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