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):

cephalexin
    n 1: an oral cephalosporin (trade names Keflex and Keflin and
         Keftab) commonly prescribe for mild to moderately severe
         infections of the skin or ears or throat or lungs or
         urinary tract [syn: cephalexin, Keflex, Keflin,
         Keftab]