modulo

21 and 84 are congruent to each other modulo 9, since both numbers leave the same remainder, 3, when divided by 9.

"A is the same as B modulo C" means A is the same as B except for differences accounted for by C.

In the Lisp programming language, the "tail recursion modulo cons" technique allows functions that would be tail recursive but for a cons call to be transformed into a tail recursive form.