What is a word opposite in meaning to another called, for instance: fast-slow, large-small?

An antonym is one of a pair of words with opposite meanings. Each word in the pair is the antithesis of the other. A word may have more than one antonym. There are three categories of antonyms identified by the nature of the relationship between the opposed meanings.

Where the two words have definitions that lie on a continuous spectrum of meaning, they are gradable antonyms. Temperature is such a continuous spectrum so hot and cold, two meanings on opposite ends of the spectrum, are gradable antonyms. Other examples include: heavy : light, fat : skinny, dark : light, young : old, early : late, empty : full, dull : interesting.

Where the meanings do not lie on a continuous spectrum and the words have no other lexical relationship, they are complementary antonyms. There is no continuous spectrum between odd and even but they are opposite in meaning and are therefore complementary antonyms. Other examples include: mortal : immortal, exit : entrance, exhale : inhale, occupied : vacant.

Where the two meanings are opposite only within the context of their relationship, they are relational antonyms. There is no lexical opposite of teacher, but teacher and pupil are opposite within the context of their relationship. This makes them relational antonyms. Other examples include: husband : wife, doctor : patient, predator : prey, teach : learn, servant : master, come : go, parent : child.