calcium carbide

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A grayish-black crystalline compound, CaC₂, obtained by heating pulverized limestone or quicklime with carbon and used to generate acetylene gas, as a dehydrating agent, and in the manufacture of graphite and hydrogen.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun inorganic chemistry A black, solid, of formula CaC2, manufactured from lime and carbon at high temperatures; reacts with water to form acetylene.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a grey salt of calcium (CaC) used in making acetylene