the action of opposing, resisting, or combating.

antagonism or hostility.

a person or group of people opposing, criticizing, or protesting something, someone, or another group.

( sometimes initial capital letter ) the major political party opposed to the party in power and seeking to replace it.

the act of placing opposite, or the state or position of being placed opposite.

the act of opposing, or the state of being opposed by way of comparison or contrast.

Logic .

the relation between two propositions that have the same subject and predicate, but which differ in quantity or quality, or in both.

the relation between two propositions in virtue of which the truth or falsity of one of them determines the truth or falsity of the other.

Astronomy . the situation of two heavenly bodies when their longitudes or right ascensions differ by 180?

The moon is in opposition to the sun when the earth is directly between them.

Astrology . the situation of two heavenly bodies or groups of heavenly bodies whose celestial longitudes differ by 180?, conducive to confrontation or revelation

an astrological aspect.

Electricity . the condition that exists when two waves of the same frequency are out of phase by one-half of a period.

Linguistics .

the relationship between any two alternative units within a linguistic system, esp. between minimally distinct phonemes.

the feature that constitutes the difference between two such units.

adjective

adversarial , adverse , antagonistic , antipathetic , opposed