The opposite (antonym) of antonym is synonym. An antonym is a word that means the opposite. A synonym is a word that means the same thing, or nearly the same thing, for a given context.
out.
Some words do not have opposites [antonyms]. The word "where" has no antonym.
he won't
Some words do not have direct opposites [antonyms]. The word "but" has no antonym. (head)