The Collaborative International Dictionary of English v.0.48:

Stalactite \Sta*lac"tite\ (-t[imac]t), n.; pl. Stalactites
   (-t[imac]ts). [Gr. stalakto`s oozing out in drops, dropping,
   fr. stala`zein to drop: cf. F. stalactite.] (Geol.)
   (a) A pendent cone or cylinder of calcium carbonate
       resembling an icicle in form and mode of attachment.
       Stalactites are found depending from the roof or sides of
       caverns, and are produced by deposition from waters which
       have percolated through, and partially dissolved, the
       overlying limestone rocks.
   (b) In an extended sense, any mineral or rock of similar form
       and origin; as, a stalactite of lava.
       [1913 Webster]

WordNet (r) 3.0 (2006):

stalactite
    n 1: a cylinder of calcium carbonate hanging from the roof of a
         limestone cave