Ternary relation

https://en.wikipedia.org/wiki/Triadic_relation

A ternary relation is a finitary relation of arity 3.

in which the number of places (arity) in the relation is 3. Ternary relations may also be referred to as 3-adic (triadic, Latin origin), 3-ary (ternary, Greek origin), 3-dimensional, or 3-place.

Just as a binary relation is formally defined as a set of pairs, i.e. a subset of the Cartesian product A × B of some sets A and B, so a ternary relation is a set of triples, forming a subset of the Cartesian product A × B × C of three sets A, B and C.

An example of a ternary relation in elementary geometry can be given on triples of points, where a triple is in the relation if the three points are collinear. Another geometric example can be obtained by considering triples consisting of two points and a line, where a triple is in the ternary relation if the two points determine (are incident with) the line.