Acting against, resisting, or opposing something; the opposite or reverse of something.
[Greek anti, against.]
Many terms in anti- are hyphenated if the next element begins with a vowel, though hyphenation is variable. The prefix is active, often forming new words that are destined to be temporary. Words may be nouns or adjectives, sometimes both.
One group represents opposition, resistance or aversion: anti-abortion, anti-apartheid, antiestablishment, anti-government, anti-nuclear, anti-Semitic, antivivisectionist. Another refers to an agent that acts against something: anti-aircraft, antibiotic, antimagnetic, antifreeze, anti-static, antiseptic. A third denotes the simple opposite of something: anticlimax, anticlockwise, Antarctic (the opposite side of the world from the Arctic), antigravity, antisocial.
Subatomic particles have mirror particles with the same mass but opposite values for other properties, such as charge; the prefix is used in the sense of ‘opposite’ to name them: antiproton, antineutrino, antimuon; the general name is antiparticle, and hypothetical matter made up entirely of such particles is antimatter.
Some words in anti-, such as anticipate, are actually formed using ante-.