The Collaborative International Dictionary of English v.0.48:

penicillin \pen`i*cil"lin\ (p[e^]n`[i^]*s[i^]l"l[i^]n), n. [From
   Penicillium, the fungus genus from which it was first
   isolated.] (Chem.)
   Any of a variety of substances having a structure containing
   a beta-lactam ring fused to a thiirane ring, to which a
   carboxyl group is attached, but most commonly interpreted as
   benzyl penicillin. They are notable as powerful
   antibacterial agents of relatively low toxicity which have
   found extensive use in medicine for treating bacterial
   infections. They are categorized as one of the classes of
   beta-lactam antibiotic. They are produced naturally by some
   fungi and bacteria, and industrial production processes
   almost invariably start from some form of the penicillin
   nucleus produced by fermentation of microorganisms. The
   fermentation products are then chemically modified to produce
   derivatives of enhanced potency, safety, or antibacterial
   spectrum. The first penicillin to see extensive use
   clinically (during World War II) was penicillin G, also
   called benzypenicillin, and commonly simply "penicillin".
   [PJC]

   Note: [hand]The characteristic portion of the structure
         characterizing a penicillin is the bicyclic structure
         3,3-Dimethyl-7-oxo-4-thia-1-azabicylo[3.2.0]heptane-2-carboxylic
         acid. The different penicillins have varying acyl
         groups forming an amide bond with a nitrogen attached
         to the 6-carbon of this nucleus. This distinguishes it
         from the other classes of beta-lactam antibiotic, the
         cephalosporins, thienamycins and sulfazecin.
         [PJC]

WordNet (r) 3.0 (2006):

penicillin G
    n 1: the penicillin that constitutes the principal component of
         many commercial antibiotics [syn: penicillin G,
         benzylpenicillin]