The Collaborative International Dictionary of English v.0.48:

Pepsin \Pep"sin\ (p[e^]p"s[i^]n), n. [Gr. pe`psis a cooking,
   digesting, digestion, fr. pe`ptein, pe`ssein, to cook,
   digest: cf. F. pepsine. Cf. Dyspepsia.] (Physiol. Chem.)
   A proteolytic enzyme (MW 34,500) contained in the secretory
   glands of the stomach. In the gastric juice it is united with
   dilute hydrochloric acid (0.2 per cent, approximately) and
   the two together constitute the active portion of the
   digestive fluid. It degrades proteins to proteoses and
   peptides, and is notable for having a very low pH optimum for
   its activity. It is the active agent in the gastric juice of
   all animals.
   [1913 Webster + PJC]

   Note: As prepared from the glandular layer of pigs' or
         calves' stomachs it constitutes an important article of
         pharmacy.
         [1913 Webster]

WordNet (r) 3.0 (2006):

pepsin
    n 1: an enzyme produced in the stomach that splits proteins into
         peptones