The Collaborative International Dictionary of English v.0.48:

Penicillinase \Pen`i*cil"lin*ase\, n. (Bioch.)
   An enzyme which destroys the antibacterial activity of
   penicillin by hydrolyzing the amide bond in the beta-lactam
   ring. Many penicillinases are known, and are produced by a
   wide variety of bacteria. The production of penicillinase is
   one of the mechanisms by which bacteria may become resistant
   to penicillins. Penicillinase production in various bacterial
   species may be induced, i.e., it may occur only when
   stimulated by the presence of penicillin in the culture
   medium, or it may be constitutive, i.e., it may occur
   whenever the cells are producing protein. Molecular weights
   of the various penicillinases tend to cluster near 50,000.
   [PJC]

WordNet (r) 3.0 (2006):

penicillinase
    n 1: enzyme produced by certain bacteria that inactivates
         penicillin and results in resistance to that antibiotic
         [syn: penicillinase, beta-lactamase]