A deep thought from Phil Plait:
The antonym of "synonym" is "antonym", and the antonym of "antonym" is "synonym"…but "synonym" has no synonym. This strikes me as not just odd but also hilariously ironic. It’s like abbreviation being such a long word, oronomatopoeia sounding nothing like its meaning. How can we not have another word for "synonym"? Are we that impoverished for words?
Commenter Chris Waigl helps out:
Few word pairs are really true 100% synonyms. There’s nearly all the time some sense or context or nuance of meaning in which one fits and the other doesn’t. Synonyms are situational: In *this particular* sentence, you could substitute X for Y because the two words can be used synonymously. Furthermore, the more specialized and technical a word, the less likely it is for there to exist a synonym. "Synonym" is such a word with a specialized technical sense.