Source : Webster's Revised Unabridged Dictionary (1913)

Pepsin \Pep"sin\, n. [Gr. ? a cooking, digesting, digestion, fr.
   ?, ?, to cook, digest: cf. F. pepsine. Cf. {Dyspepsia}.]
   (Physiol. Chem.)
   An unorganized proteolytic ferment or enzyme 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 is the active agent in the
   gastric juice of all animals.

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

Source : WordNet®

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