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]