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. Antonyms are of four types:

• Gradable antonyms are two ends of the spectrum (slow and fast) but can have variations.
• Complementary antonyms are pairs that express absolute opposites, like mortal and immortal.
• Relational antonyms are pairs in which one describes a relationship between two objects and the other describes the same relationship when the two objects are reversed, such as parent and child, teacher and student, or buy and sell.
• Auto-antonyms are the same words that can mean the opposite of themselves under different contexts or having separate definitions:
• enjoin (to prohibit, issue injunction; to order, command)
• fast (moving quickly; fixed firmly in place)
• cleave(to split; to adhere)
• sanction(punishment, prohibition; permission)
• stay (remain in a specific place, postpone; guide direction, movement)

Although the word antonym was only coined by philosophers in the 19th century, such relationships are a fundamental part of a language, in contrast to synonyms, which are a result of history and drawing of fine distinctions, or homonyms, which are mostly etymological accidents or coincidences.

Languages often have ways of creating antonyms as an easy extension of lexicon. An example is the English prefixes in- and un-. Unreal is the antonym of real and indocile is of docile.

Some planned languages abundantly use such devices to reduce vocabulary multiplication. Esperanto has mal- (compare bona = "good" and malbona = "bad"), Damin has kuri- (tjitjuu "small", kuritjitjuu "large") and Newspeak has un- (as in ungood, "bad").