@MartinSleziak Perhaps foundations? congruences could be replaced by congruence-relations, I think -- the core question is about $\sim_d$ being a congruence for multiplication.

Some suggestions for an amalgamate of "congruence" and "modular-arithmetic": number-congruence, congruences-and-moduli, congruences-and-remainders. I guess I like the last one best: it's most likely to be found by new users ("moduli" is a word only encountered at a much later stage).

Meta post updated.

IIRC the synonyms share the tag-excerpt and the tag-wiki.

So it would have to go into the tag-excerpt for modular-arithmetic.

@MartinSleziak Ok. It just needs to be mentioned.