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Glossary of Terms

This document is a list of terms that you might encounter in the ontology world. It is not an exhaustive list and will continue to evolve. Please create a ticket if there is a term you find missing or a term you encounter that you do not understand, and we will do our best to add them. This list is not arranged in any particular order. Please use the search function to find terms.

Acknowledgement: Many terms are taken directly from OAK documentation with the permission of Chris Mungall. Many descriptions are also taken from


This term is frequently ambiguous. It can refer to Text Annotation, OWL Annotation, or Association.


Annotation properties are OWL axioms that are used to place annotations on individuals, class names, property names, and ontology names. They do not affect the logical definition unless they are used as a "shortcut" that a pipeline expands to a logical axiom.

Anonymous Ancestor

An accumulation of all of the superclasses from ancestors of a class.

Anonymous Individual

If an individual is not expected to be used outside an ontology, one can use an anonymous individual, which is identified by a local node ID rather than a global IRI. Anonymous individuals are analogous to blank nodes in RDF.


Application Programming Interface. An intermediary that allows two or more computer programs to communicate with each other. In ontologies, this usually means an Endpoint in which the ontology can be programmatically accessed.

Application Ontology

Usually refers to a Project Ontology.


Axioms are statements that are asserted to be true in the domain being described. For example, using a subclass axiom, one can state that the class a:Student is a subclass of the class a:Person. (Note: in OWL, there are also annotation axioms which does not apply any logical descriptions)


An Ontology Repository that is a comprehensive collection of multiple biologically relevant ontologies.

Controlled Vocabulary

Standardized and organized arrangements of words and phrases that provide a consistent way to describe data. A controlled vocabulary may or may not include definitions. Ontologies can be seen as a controlled vocabulary expressed in an ontological language which includes relations.


An OWL entity that formally represents something that can be instantiated. For example, the class "heart".


A CURIE is a compact URI. For example, CL:0000001 expands to For more information, please see


curies is a Python package for working with prefix maps and extended prefix maps and converting between CURIEs and URIs.

Data Model

An abstract model that organizes elements of data and standardizes how they relate to one another.


dataProperty relate OWL entities to literal data (e.g., strings, numbers, datetimes, etc.) as opposed to ObjectProperty which relate individuals to other OWL entities. Unlike AnnotationProperty, dataProperty axioms fall on the logical side of OWL and are hence useable by reasoners.


Datatypes are OWL entities that refer to sets of data values. Thus, datatypes are analogous to classes, the main difference being that the former contain data values such as strings and numbers, rather than individuals. Datatypes are a kind of data range, which allows them to be used in restrictions. For example, the datatype xsd:integer denotes the set of all integers, and can be used with the range of a dataProperty to state that the range of said dataProperty must be an integer.

Description Logic

Description Logics (DL) are a family of formal knowledge representation languages. It provides a logical formalism for ontologies and is what OWL is based on. DL querying can be used to query ontologies in Protege.


Domain, in reference to a dataProperty or ObjectProperty, refers to the restriction on the subject of a triple - if a given property has a given class in its domain this means that any individual that has a value for the property, will be inferred to be an instance of that domain class. For example, if John hasParent Mary and Person is listed in the domain of hasParent, then John will be inferred to be an instance of Person.


Dead Simple Ontology Design Patterns. A templating system for ontologies with well-documented patterns and templates.


A typed, directed link between Nodes in a knowledge graph. Translations of OWL into Knowledge graphs vary, but typically edges are generated for simple triples, relating two individuals or two classes via an AnnotationProperty or ObjectProperty and simple existential restrictions (A SubClassOf R some B), with the edge type corresponding to the property.


Where an API interfaces with the ontology.

Existential Restriction

A relationship between two classes, A R (some) B, that states that all individuals of class A stand in relation R to at least one individual of class B. For example, neuron has_part some dendrite states that all instances of neuron have at least one individual of type dentrite as a part. In Manchester syntax, the keyword 'some' is used to denote existential restrictions and is interpreted as "there exists", "there is at least one", or "some". See documentation on classifications for more details.

Extended Prefix Map

Extended prefix maps (EPMs) address the limitations in expressiveness of prefix maps by including explicit fields for CURIE prefix synonyms and URI prefix synonyms in addition to explicit fields for the preferred CURIE prefix and URI prefix. An abbreviated example (just containing an entry for ChEBI) looks like:

        "prefix": "CHEBI",
        "uri_prefix": "",
        "prefix_synonyms": ["chebi"],
        "uri_prefix_synonyms": [

Functional Syntax

An official syntax of OWL (others are RDF-XML and OWL-XML) in which each line represents and axiom (although things get a little more complex with axiom annotations, and axioms use prefix syntax (order = relation (subject, object)). This is in contrast to in-fix syntax (e.g. Manchester syntax) (order = subject relation object). Functional syntax is the preferred syntax for editor files maintained on GitHub, because it can be safely diff'd and (somewhat) human readable.


Formally a graph is a data structure consisting of Nodes and Edges. There are different forms of graphs, but for our purposes an ontology graph has all Terms as nodes, and relationships connecting terms (is-a, part-of) as edges. Note the concept of an ontology graph and an RDF graph do not necessarily fully align - RDF graphs of OWL ontologies employ numerous blank nodes that obscure the ontology structure.


An OWL entity that represents an instance of a class. For example, the instance "John" or "John's heart". Note that instances are not commonly represented in ontologies. For instance, "John" (an instance of person) or "John's heart" (an instance of heart).

Information Content

A measure of how informative an ontology concept is; broader concepts are less informative as they encompass many things, whereas more specific concepts are more unique. This is usually measured as -log2(Pr(term)). The method of calculating the probability varies, depending on which predicates are taken into account (for many ontologies, it makes sense to use part-of as well as is-a), and whether the probability is the probability of observing a descendant term, or of an entity annotated using that term.


A programmatic abstraction that allows us to focus on what something should do rather than how it is done.

Jaccard Similarity

A measures of the similarity between two sets of data to see which members are shared and distinct.


Knowledge Graph Change Language (KGCL) is a data model for communicating desired changes to an ontology. It can also be used to communicate differences between two ontologies. See KGCL docs.

Knowledge Graph

A network of real-world entities (i.e., objects, events, situations, and concepts) that illustrates the relationships between them. Knowledge graphs (in relation to ontologies) are thought of as real data built using an ontology as a framework.


Usually refers to a human-readable text string corresponding to the rdfs:label predicate. Labels are typically unique per ontology. In OBO Format and in the bio-ontology literature, labels are sometimes called Names. Sometimes in the machine learning literature, and in databases such as Neo4J, "label" actually refers to a Category.


Lutra is the open source reference implementation of the OTTR templating language.


A means of linking two resources (e.g. two ontologies, or an ontology and a database) together. Also see SSSOM


The process of making inferred axioms explicit by asserting them.


Usually synonymous with Label, but in the formal logic and OWL community, "Name" sometimes denotes an Identifier

Named Individual

An Individual that is given an explicit name that can be used in any ontology to refer to the same object; named individuals get IRIs whereas anonymous individuals do not.


Terms represented in a graph


The "right" side of a Triple.


An owl entity that is used to related 2 individuals ('my left foot' part_of 'my left leg') or two classes ('foot' part_of some leg) or an individual and a class ('the neuron depicted in this image' (is) has_soma_location some 'primary motor cortex. More rarely it is used to define a class in terms of some individual (the class 'relatives of Shawn' related_to Value Shawn.


Open Biological and Biomedical Ontology. This could refer to the OBO Foundry (e.g. OBO ontologies = ontologies that follow the standards of the OBO Foundry) or OBO Format

OBO Format

A serialization format for ontologies designed for easy viewing, direct editing, and readable diffs. It is popular in bioinformatics, but not widely used or known outside the genomics sphere. OBO is mapped to OWL, but only expresses a subset, and provides some OWL abstractions in a more easy to understand fashion.


Ontology Lookup Service. An Ontology Repository that is a curated collection of multiple biologically relevant ontologies, many from OBO. OLS can be accessed with this link


A flexible concept loosely encompassing any collection of OWL entities and statements or relationships connecting them.


Ontology Development Kit. A toolkit and docker image for managing ontologies.

Ontology Library

The systems or platform where various types of ontologies are stored from different sources and provide the ability to data providers and application developers to share and reuse the ontologies.

Ontology Repository

A curated collection of ontologies.


Reasonable Ontology Templates. A system for composable ontology templates and documentation.


Web Ontology Language. An ontology language that uses constructs from Description Logic. OWL is not itself an ontology format, it can be serialized through different formats such as Functional Syntax, and it can be mapped to :RDF and serialized via an RDF format.

OWL Annotation

In the context of OWL, the term Annotation means a piece of metadata that does not have a strict logical interpretation. Annotations can be on entities, for example, Label annotations, or annotations can be on Axioms.


A java-based API to interact with OWL ontologies. Full documentation can be found at

OWL Entity

OWL Entities, such as classes, properties, and individuals, are identified by IRIs. They form the primitive terms of an ontology and constitute the basic elements of an ontology. For example, a class a:Person can be used to represent the set of all people. Similarly, the object property a:parentOf can be used to represent the parent-child relationship. Finally, the individual a:Peter can be used to represent a particular person called "Peter". The following is a complete list of types of OWL Entities:


An OWL entity that represents the type of a Relationship. Typically corresponds to an ObjectProperty in OWL, but this is not always true; in particular, the is-a relationship type is a builtin construct SubClassOf in OWL Examples:

  • is-a
  • part-of (BFO:0000050)

Prefix Map

A prefix map is a dictionary data structure where keys represent CURIE prefixes and their associated values represent URI prefixes. Ideally, these are constrained to be bijective (i.e., no duplicate keys, no duplicate values), but this is not always done in practice. Here’s an abbreviated example prefix map describing OBO Foundry ontologies:

  "CHEBI": "",
  "MONDO": "",
  "GO": ""

Project Ontology

An ontology that is specific to a project and does not necessarily have interoperability with other ontologies in mind.


An Ontology Library for parsing obo and owl files.


An OWL entity that represents an attribute or a characteristic of an element. In OWL, properties are divided into disjoint categories:


A typical ontology development tool used by ontology developers in the OBO-sphere. Full documentation can be found at


Range, in reference to a dataProperty or ObjectProperty, refers to the restriction on the object of a triple - if a given property has a given class in its domain this means that any individual that has a value for the property (i.e. is the subject of a relation along the property), will be inferred to be an instance of that domain class. For example, if John hasParent Mary and Person is listed in the domain of hasParent, then John will be inferred to be an instance of Person.


A datamodel consisting of simple Subject predicate Object Triples organized into an RDF Graph.


A python library to interact with RDF data. Full documentation can be found at


An ontology tool that will perform inference over an ontology to yield new axioms (e.g. new Edges) or to determine if an ontology is logically coherent.


A Relationship is a type connection between two OWL entities. The first element is called the subject, and the second one the Object, with the type of connection being the Relationship Type. Sometimes Relationships are equated with Triples in RDF but this can be confusing, because some relationships map to multiple triples when following the OWL RDF serialization. An example is the relationship "finger part-of hand", which in OWL is represented using a Existential Restriction that maps to 4 triples.

Relationship Type

See predicate


A toolkit for transforming and interacting with ontologies. Full documentation can be found at

Semantic Similarity

A means of measuring similarity between either pairs of ontology concepts, or between entities annotated using ontology concepts. There is a wide variety of different methods for calculating semantic similarity, for example Jaccard Similarity and Information Content based measures.

Semantic SQL

Semantic SQL is a proposed standardized schema for representing any RDF/OWL ontology, plus a set of tools for building a database conforming to this schema from RDF/OWL files. See Semantic-SQL


The standard query language and protocol for Linked Open Data on the web or for RDF triplestores - used to query ontologies.


Simple Standard for Sharing Ontological Mappings (


The "left" side of a Triple.


A named collection of elements, typically grouped for some purpose. In the ODK/OBO world, there is a standard annotation property and pattern for this, for more information, see the subset documentation.


Usually used to mean Class and Individuals, however sometimes used to refer to wider OWL entities.

Text Annotation

The process of annotating spans of texts within a text document with references to ontology terms, or the result of this process. This is frequently done automatically. The Bioportal implementation provides text annotation services.


A set of three entities that codifies a statement about semantic data in the form of Subject-predicate-Object expressions (e.g., "Bob is 35", or "Bob knows John"). Also see Relationship.


A purpose-built database for the storage and retrieval of triples through semantic queries. A triple is a data entity composed of subject–predicate–object, like "Bob is 35" or "Bob knows Fred".


An integrated OBO ontology Triplestore and a Ontology Repository, with merged set of mutually referential OBO ontologies (see the ubergraph github for list of ontologies included), that allows for SPARQL querying of integrated OBO ontologies.


A Uniform Resource Indicator, a generalization of URL. Most people think of URLs as being solely for addresses for web pages (or APIs) but in semantic web technologies, URLs can serve as actual identifiers for entities like OWL entities. Data models like OWL and RDF use URIs as identifiers. In OAK, URIs are mapped to CURIE