Real Dictionary

penicillin in American English
any of a group of isomeric, antibiotic compounds with the general formula C9H11N2O4SR, obtained from the filtrates of certain molds (esp., Penicillium notatum and P. chrysogenum) or produced synthetically
noun: 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 again

penicillin in British English
noun: any of a group of antibiotics with powerful bactericidal action, used to treat many types of infections, including pneumonia, gonorrhoea, and infections caused by streptococci and staphylococci: originally obtained from the fungus Penicillium, esp P. nota

penicillin in the Pharmaceutical Industry
noun: Penicillin is one of any of a group of antibiotics that kills bacteria and is used to treat many types of infections, including pneumonia and gonorrhea.