The word modulus has several different meanings in mathematics with respect to complex numbers, congruences, elliptic integrals, quadratic invariants, sets, etc. The modulus of a congruence a congruent b (mod m) is the number m. It is the "base" with respect to which a congruence is computed (i.e., m gives the number of multiples of a that are "thrown out"). For example, when computing the time of day using a 12-hour clock obtained by adding four hours to 9:00, the answer, 1:00, is obtained by taking 9 + 4 congruent 1 (mod 12) (i.e., adding the hours with modulus 12).

complex modulus | congruence | elliptic modulus | quadratic invariant modulus | set modulus