Set of numbers that have the same remainder from division by n.
These numbers are called congruent modulo n.
Example
If we group the elements of the set H = {2, 3, 4, 6, 7, 8, 10, 11, 12} based on the relation R: … has the same remainder as… when divided by 4, we find these three classes of equivalence: {2, 6, 10}, {3, 7, 11} and {4, 8, 12}.