The Collaborative International Dictionary of English v.0.48:

papaya \pa*pa"ya\, n. [Prob. from the native name in the West
   Indies; cf. Sp. papayo papaw, papaya the fruit of the papaw.]
   1. (Bot.) A tree (Carica Papaya) of tropical America,
      belonging to the order Passiflore[ae]; called also
      papaw and pawpaw. It has a soft, spongy stem, eighteen
      or twenty feet high, crowned with a tuft of large,
      long-stalked, palmately lobed leaves. The milky juice of
      the plant is said to have the property of making meat
      tender.
      [1913 Webster +PJC]

   2. The fruit of the papaya tree; it is a dull orange-colored,
      melon-shaped fruit, which is eaten both raw and cooked or
      pickled. The fruit contains papain, a protease.
      [1913 Webster + PJC]

The Collaborative International Dictionary of English v.0.48:

Papain \Pa*pa"in\, n. [From Papaw.] (Physiol. Chem.)
   A proteolytic ferment, like trypsin, present in the juice of
   the green fruit of the papaw (Carica Papaya) of tropical
   America.
   [1913 Webster]

WordNet (r) 3.0 (2006):

papain
    n 1: a proteolytic enzyme obtained from the unripe papaya; used
         as a meat tenderizer