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Disambiguation: The term 'Relation' as used in the literature may refer to one of the following, depending on the context:

  • Instance-level entity
  • Type-level entity, used interchangeably with, mainly, relationship (e.g., in Entity-Relationship diagrams), association (UML class diagrams), and object property (in OWL)
  • Its definitions in mathematics, notably logic

D1 (as instance-level entity)

D2 (as type-level entity)

  • A relation is an entity that asserts a (meaningful) connection between two or more other entities, where the entities are generally denoted with class/ concept/ universal.


1. [Natural Language] The way in which two or more people or things are connected. (
2. Relations hold between things, or, alternatively, relations are borne by one thing to other things, or, another alternative paraphrase, relations have a subject of inherence whose relations they are and termini to which they relate the subject. ( MacBride, 2016 ])
3. [BFO2.0] The manner in which two or more entities are associated or connected together. BFO recognizes three basic types of relation: connecting universal to universal, universal to particular, and particular to particular. ([ Arp et al., 2015 ])
3. A relationship set, R, is a mathematical relation among n entities, each taken from an entity set: {(e1, e2, ..., en) | e1 ∈ E1, e2 ∈ E2, ..., en ∈ En}, and each tuple of entities, (e1, e2, ..., en), is a relationship. ([ Chen, 1976 ])


Closely Related Terms

  • Domain and range/codomain, also called, more generally, relata
  • Relational properties / property characteristics
  • The entities mentioned in the definitions above may be one and the same; if that is the case, then that relation is called a recursive relation.