|Definition:|| binary relation ℛ between elements a and b of a given set having the following properties: |
- reflexivity: aℛa,
- symmetry: if aℛb then bℛa,
- transitivity: if aℛb and bℛc then aℛc for any elements a, b and c of the given set
Note 1 to entry: Examples are the equality of elements of a set, the parallelism of straight lines in a point space, the relation between integers whose difference is even.