Source : Webster's Revised Unabridged Dictionary (1913)

Cocoon \Co*coon"\, n. [F. cocon, dim. of coque shell of egge and
   insects, fr. L. concha mussel shell. See {Conch}.]
   1. An oblong case in which the silkworm lies in its chrysalis
      state. It is formed of threads of silk spun by the worm
      just before leaving the larval state. From these the silk
      of commerce is prepared.

   2. (Zo["o]l.)
      (a) The case constructed by any insect to contain its
          larva or pupa.
      (b) The case of silk made by spiders to protect their
          eggs.
      (c) The egg cases of mucus, etc., made by leeches and
          other worms.

Source : WordNet®

cocoon
     n : silky envelope spun by the larvae of many insects to protect
         pupas and by spiders to protect eggs
     v 1: retreat as if into a cocoon, as from an unfriendly
          environment; "Families cocoon around the T.V. set most
          evenings"; "She loves to stay at home and cocoon"
     2: wrap in or as if in a cocoon, as for protection