The Collaborative International Dictionary of English v.0.48:

Icicle \I"ci*cle\, n. [OE. isikel, AS. [imac]sgicel; [imac]s ice
   + gicel icicle; akin to Icel. j["o]kull; cf. Gael. eigh ice,
   Ir. aigh.]
   A pendent, and usually conical, mass of ice, formed by
   freezing of dripping water; as, the icicles on the eaves of a
   house.
   [1913 Webster]

WordNet (r) 3.0 (2006):

icicle
    n 1: ice resembling a pendent spear, formed by the freezing of
         dripping water