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1670s, Englished from Modern Latin stalactites (used 1654 by Olaus Wormius), from Greek stalaktos "dripping," from stalassein "to trickle," from PIE root *stag- "to seep, drip, drop" (cf. German stallen, Lithuanian telziu "to urinate").
A cylindrical or conical mineral deposit projecting downward from the roof of a cave or cavern, formed by the dripping of water saturated with minerals. Stalactites form gradually as the minerals precipitate out of the saturated water. They usually consist of calcite but can also consist of other minerals. Compare stalagmite.