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
see also:
beta-lactam beta-lactam penicillin cephalosporin carbapenem beta-lactam antibiotic