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An implicant (Boolean product term) which is called "prime" because none of its proper factors is itself an implicant.
Etymology from Wiktionary, CC BY-SA 4.0.
prime implicant
Today it means: A group of related 1's (implicant) on a Karnaugh map which is not subsumed by any other implicant in the same map. Equivalently (in terms of Boolean algebra), a product term which is a "minimal" implicant in the sense that removing any of its literals will yield a product term which is not an implicant (but beware: on a Karnaugh map it would appear "maximal"). Full definition →