The ontology has textual definitions for the majority of its classes and for top level terms in particular.


A textual definition provides a human-readable understanding about what is a member of the associated class. Textual definitions are, optimally, in concordance with associated machine-readable logical definitions (the latter of which are OPTIONAL).

Recommendations and Requirements

Textual definitions MUST be unique within an ontology (i.e. no two terms should share a definition). Textual definitions SHOULD follow Aristotelian form (e.g. “a B that Cs” where B is the parent and C is the differentia), where this is practical.

For terms lacking textual definitions, there should be evidence of implementation of a strategy to provide definitions for all remaining undefined terms. In lieu of textual definitions, there can be elucidations when the term can not be rigorously defined. Note that textual definitions can be programmatically generated from logical definitions, if available (see In addition, Dead Simple Ontology Design Patterns (DOSDPs) can be used to generate both textual and logical definitions. DOSDPs are design specifications, written in YAML format, that specify structured text definitions and logical definitions for groups of ontology terms. These are widely used in many OBO Foundry ontologies, such as Mondo and uPheno. For some example patterns, see Mondo patterns and uPheno patterns.

Textual definitions should agree with logical definitions and vice versa. This is important for two reasons: (1) Reasoners classify terms solely based on logical definitions, while humans predominantly classify terms based on textual definitions, and mismatches between the two can cause unexpected misclassification; and (2) Curators could create incorrect annotations. An example of mismatched definitions: spermatocyte

Text definition: "A male germ cell that develops from spermatogonia."

Logical definition (that mismatches the textual def): 
= 'male germ cell' and ('capable of' some 'spermatocyte division')

The logical definition could be revised to:

Logical definition (that matches the textual def): 
= ‘male germ cell’ and (‘develops from’ some 'spermatogonium')

While both logical definitions can be used to define the class, one better fits with the textual definition than the other.

Note that it’s permissible to not to have a logical definition if the class is fuzzy or the axioms/relations can’t be composed equivalence axioms.

Terms often benefit from examples of usage, as well as editor notes about edge cases and the history of the term, but these should be included as separate annotations and not in the definition.

Instances, such as organizations or geographical locations, can benefit from definitions although it is understood that definitions for instances are not required. It is recognized that OBO format (e.g., versions 1.2 and 1.4) does not allow this as an option.


Textual definitions should be identified using the annotation property: ‘definition’ The source of the definition should be provided using the annotation property ‘definition source’, or as an axiom annotation on the definition assertion.

An example of providing source in an axiom annotation:

<> rdf:type owl:Class ;
                                            <> "Any complex formed of proteins that act in nucleotide-excision repair."@en ;
                                            rdfs:label "nucleotide-excision repair complex"^^xsd:string .

[ rdf:type owl:Axiom ;
   owl:annotatedSource <> ;
   owl:annotatedProperty <> ;
   owl:annotatedTarget "Any complex formed of proteins that act in nucleotide-excision repair."@en ;
   <> "PMID:10915862"^^xsd:string
 ] .

this corresponds to the obo format:

id: GO:0000109
name: nucleotide-excision repair complex
def: "Any complex formed of proteins that act in nucleotide-excision repair." [PMID:10915862]


Class: reproductive shoot system
Term IRI:
Definition: A shoot system (PO:0009006) in the sporophytic phase that has as part at least one sporangium (PO:0025094).
Logical definition:

intersectionOf: shoot system
intersectionOf: participates_in some reproductive shoot system development stage

Class: chromatography device
Term IRI:
Definition: A device that facilitates the separation of mixtures. The function of a chromatography device involves passing a mixture dissolved in a “mobile phase” through a stationary phase, which separates the analyte to be measured from other molecules in the mixture and allows it to be isolated.
Definition source:
Logical definition:

intersectionOf: device
intersectionOf: has_function some material separation function


  • No definition
  • Circular/Self-referential definition “A chromatography device is a device that uses chromatography” when chromatography is not defined elsewhere

Criteria for Review

Each definition MUST be unique. Each entity MUST NOT have more than one textual definition (tagged using IAO:0000115). Textual definitions SHOULD be provided for most terms, and for top level terms especially.

This check is automatically validated.

Feedback and Discussion

To suggest revisions or begin a discussion pertaining to this principle, please create an issue on GitHub.

To suggest revisions or begin a discussion pertaining to the automated validation of this principle, please create an issue on GitHub.