A variation of the word antonym, in a manner similar to the way synonym and synonymous are related. When several words are synonyms, they mean approximately the same thing. However, when they are synonymous, they mean exactly the same thing. Similarly, there must be a word that means exactly opposite, to pair with "antonym" which only means approximately opposite. Thus, antonymous words are words that are exact polar opposites in meaning.
Black and white are antonymous.