Word with the opposite meaning to another: hot/cold. It's the opposite of a synonym - e.g. tin / can, though it is often difficult to replace one word successfully with another in any particular context.
A word or term which is opposite to another, such as flammable and non-flammable, as opposed to synonymous (i.e., two words or terms having the same or nearly the same meaning, such as joyful and glad).
Antonyms, from the Greek anti ("opposite") and onoma ("name") are word pairs that are opposite in meaning, such as hot and cold, obese and skinny, and up and down. Words may have different antonyms, depending on the meaning. Both long and tall are antonyms of short.