21 and 84 are congruent to each other modulo 9, since both numbers leave the same remainder, 3, when divided by 9.
preposition Except for differences accounted for by.
"A is the same as B modulo C" means A is the same as B except for differences accounted for by C.
preposition (The addition of quotations indicative of this usage is being sought:) With due allowance for (a specified exception or particular detail).
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.
noun The operation or function that returns the remainder of one number divided by another.