What's the difference between "synonym for" and "synonym of"?

Question

For instance, if I need to assert that "a is a synonym for|of b", which preposition may be relevant here - 'of' or 'for'? Also, which usage is grammatically correct?

Answer 1

For and of are slightly different in their meanings.

The following are from the Wiktionary.

• Of. From Middle English of, from Old English of (“from, out of”), an unstressed form of af, æf (“from, off, away”), from Proto-Germanic *ab (“away; away from”). Doublet of off, which is the stressed descendant of the same Old English word.

• For. From Middle English for, from Old English for (“for, on account of, for the sake of, through, because of, owing to, from, by reason of, as to, in order to”), from Proto-Germanic *furi (“for”), from Proto-Indo-European *preh₂-.

Your choice of of or for can give the reader a sense of what you intend to do with the synonym you have identified.

For example, if your ultimate intent is to substitute the synonym for the original, for is a better choice. If you are simply remarking on the equivalence of the words, of may convey this better.