The Collaborative International Dictionary of English v.0.48:

Pupa \Pu"pa\, n.; pl. L. Pup[ae], E. Pupas. [L. pupa girl.
   doll, puppet, fem. of pupus. Cf. Puppet.]
   1. (Zool.) Any insect in that stage of its metamorphosis
      which usually immediately precedes the adult, or imago,
      stage.
      [1913 Webster]

   Note: Among insects belonging to the higher orders, as the
         Hymenoptera, Diptera, Lepidoptera, the pupa is inactive
         and takes no food; in the lower orders it is active and
         takes food, and differs little from the imago except in
         the rudimentary state of the sexual organs, and of the
         wings in those that have wings when adult. The term
         pupa is sometimes applied to other invertebrates in
         analogous stages of development.
         [1913 Webster]

   2. (Zool.) A genus of air-breathing land snails having an
      elongated spiral shell.
      [1913 Webster]

   Coarctate pupa, or Obtected pupa, a pupa which is incased
      in the dried-up skin of the larva, as in many Diptera.

   Masked pupa, a pupa whose limbs are bound down and partly
      concealed by a chitinous covering, as in Lepidoptera.
      [1913 Webster]

WordNet (r) 3.0 (2006):

pupa
    n 1: an insect in the inactive stage of development (when it is
         not feeding) intermediate between larva and adult