Relations are entities that glue together other entities. Every relation has a number of relata as arguments, which are connected or related by it. The number of a relation’s arguments is called its arity. As much as an unary property such as being Red, properties of higher arities such as being married-to, being heavier-than are universals, since they can be predicated of a multitude of individuals. Relations can be classified according to the types of their relata. There are relations between sets, between individuals, and between universals, but there are also cross-categorical relations, for example, between urelements and sets or between sets and universals. We divide relations into two broad categories, called Material and Formal relations. Formal relations hold between two or more entities directly without any further intervening individual. Examples of formal relations are:

  • 5 is greater than 3
  • this day is part of this month
  • N is subset of Q

but also the relations of instantiation, inherence, quale of a quality, association, existential dependence, among others – … relations that form the mathematical superstructure of our framework. Material relations, conversely, have material structure on their own and include examples such as:

  • employments
  • kisses
  • enrollments
  • flight connections
  • commitments

The relata of a material relation are mediated by individuals that are called relators. Relators are individuals with the power of connecting entities:

  • a flight connection, for example, founds a relator that connects airports
  • an enrollment is a relator that connects a student with an educational institution

Quoted from:

GUIZZARDI, Giancarlo. Ontological Foundations for Structural Conceptual Models. Enschede: CTIT, Telematica Instituut, 2005.