A number that is not a prime number; a composite number or one that has factors other than itself and one.
From anti- (opposite of) + prime (in mathematics, a number divisible only by one and itself). This is a mathematical term that emerged in 20th-century number theory to provide a convenient label for non-prime numbers.
Mathematicians could just say 'composite number,' but 'antiprime' is useful shorthand that shows how mathematicians think in opposites—once you define primes as special, everything else becomes their opposite. It's the linguistic equivalent of how physicists use 'antiparticles.'
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