Which prefix means 'not'?

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Prefixes that negate meaning attach to the front of a root word. The one that conveys not or opposite is un-. By adding un- to a base word you typically create the opposite or a negative sense: happy becomes unhappy, possible becomes impossible, known becomes unknown. The other prefixes don’t express negation: re- indicates doing again or returning, semi- means half or partly, and trans- means across or beyond. So un- is the best choice for meaning not.