Antonym, a word that means the opposite of a word

Opposite category or dual category, in category theory

Additive inverse, in mathematics, taking the negative ("opposite") of a number

Opposition (astronomy), a term describing the position of a celestial body Opposition (chess), a term describing the position of the kings relative to each other Opposition proceeding, an administrative process available under some patent or trademark...

In trigonometry on a right angle triangle, opposite can mean the opposite side, or side not forming the angle being investigated.