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.
   [1913 Webster]

   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
       slang]
       [PJC]