methylene blue

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

• noun A heterocyclic aromatic compound, C₁₆H₁₈N₃SCl·3H₂O, consisting of dark green crystals that form a deep blue solution when dissolved in water. It is used as an antidote for cyanide poisoning, an analytical indicator, and a bacteriological stain.

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

• noun organic chemistry A heterocyclic aromatic chemical compound with many uses in biology and chemistry.

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

• noun a dark green dye used as a stain, an antiseptic, a chemical indicator, and an antidote in cyanide poisoning