A stalactite (UK: /ˈstæl.əkˌtaɪt/, US: /stəˈlækˌtaɪt/; from the Greek 'stalaktos' ('dripping') via

stalassein ('to drip'), is a mineral formation that hangs from the ceiling of caves, hot springs, or man-made structures such as bridges and mines. Any material that is soluble and that can be deposited as a colloid, or is in suspension, or is capable of being melted, may form a stalactite. Stalactites may be composed of lava, minerals, mud, peat, pitch, sand, sinter, and amberat (crystallized urine of pack rats). A stalactite is not necessarily a speleothem, though speleothems are the most common form of stalactite because of the abundance of limestone caves.

The corresponding formation on the floor of the cave is known as a stalagmite. Mnemonics have been developed for which word refers to which type of formation; one is that stalactite has a C for "ceiling", and stalagmite has a G for "ground".

 

Note:   The above text is excerpted from the Wikipedia article Stalactite, which has been released under the GNU Free Documentation License.