lactam

n. [ Lactone + amido. ] (Chem.) One of a series of anhydrides of an amido type, analogous to the lactones, as oxindol; a cyclic amide. [ 1913 Webster ]

beta-lactam or
β-lactam, (a) a lactam in which the amide bond is contained within a four-membered ring, which includes the amide nitrogen and the carbonyl carbon. (b) an antibiotic containing a beta-lactam, such as a penicillin, cephalosporin, or carbapenem; also called a beta-lactam antibiotic. [ informal, laboratory slang ] [ PJC ]