The Collaborative International Dictionary of English v.0.48:
Lactam \Lac"tam\, n. [Lactone + amido.] (Chem.)
One of a series of anhydrides of an amido type, analogous to
the lactones, as oxindol; a cyclic amide.
beta-lactam or [beta]-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