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]

The Collaborative International Dictionary of English v.0.48:

Cephalosporin \Ceph"a*lo*spor`in\
   (s[e^]f"[.a]*l[-o]*sp[=o]r`[i^]n), n. [from Cephalosporium, a
   fungus producing the first of the series discovered.] (Chem.)
   any of a class of chemical substances, some of which have
   therapeutically useful antibacterial activity, whose
   structure contains a beta-lactam ring fused to a six-membered
   ring containing a sulfur and a nitrogen atom. The first of
   the series, cephalosporin C, was discovered by G. Brotzu in
   1955 in the culture broth of a Cephalosporium species found
   off the coast of Sardinia. Other cephalosporins have been
   found to be produced by species of soil bacteria
   (actinomycetes). Many semisynthetic analogs have been tested
   for antibacterial effect, and several of them have found use
   as important clinically useful antibacterial agents, some of
   which may be taken orally for treatment of bacterial
   infections. The cephalosporins are the second class of
   beta-lactam antibiotic to be discovered, the first being the
   penicillins and more recent classes being the
   thienamycins and sulfazecins. The cephamycins are a
   variant of cephalosporins with a methoxyl group on the
   beta-lactam ring, rendering them more resistant to
   penicillinases. Among the cephalosporins which have been
   found clinically useful are cephalexin, cephaloridine,
   and cephalothin.
   [PJC]

WordNet (r) 3.0 (2006):

cephalosporin
    n 1: one of several broad spectrum antibiotic substances
         obtained from fungi and related to penicillin (trade names
         Mefoxin); addition of side chains has produced
         semisynthetic antibiotics with greater antibacterial
         activity [syn: cephalosporin, Mefoxin]