A prefix whose basic meaning is "against." It is used to form adjectives that mean "counteracting" (such as antiseptic, preventing infection). It is also used to form nouns referring to substances that counteract other substances (such as antihistamine, a substance counteracting histamine), and nouns meaning "something that displays opposite, reverse, or inverse characteristics of something else" (such as anticyclone, a storm that circulates in the opposite direction from a cyclone). Before a vowel it becomes ant-, as in antacid.
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