Definition of Penicillin 1. Any of various antibiotics obtained from Penicillium molds (or produced synthetically) and used in the treatment of various infections and diseases Noun Semanticaly linked words with "penicillin" Hyponims for word "penicillin" etymology carboxypenicillin etymology nonpenicillin etymology penicillinase etymology penicylinder etymology penicilina [Portuguese] is derived from benzathine penicillin is derived from benzylpenicillin is derived from penicillin G is derived from penicillin V is derived from penicillinic acid is derived from penicillins is derived from phenoxymethylpenicillin is derived from procaine penicillin