/pen'euh sil"in/ , n. Pharm.
any of several antibiotics of low toxicity, produced naturally by molds of the genus Penicillium and also semisynthetically, having a bactericidal action on many susceptible Gram-positive or Gram-negative cocci and bacilli, some also being effective against certain spirochetes.
[ 1925-30; PENICILL(IUM) + -IN 2 ]