noun Download 🔗Share Any of a group of narrow-spectrum antibiotics obtained from Penicillium molds or synthesized; they have a beta-lactam structure; most are active against gram-positive bacteria and used in the treatment of various infections and diseases. Examples : "The doctor prescribed penicillin for my son's ear infection, hoping the antibiotic would quickly kill the bacteria. " medicine biology chemistry biochemistry microorganism disease substance Chat with AIVocabulary GamePractice Reading