a bluish-green aniline dye, CHNClS·3HO, used as a bacteriologic stain, an antidote in cyanide poisoning, etc.
See methylene blue in American Heritage Dictionary 4
noun
A basic aniline dye, C16H18N3SCl · 3H2O, that forms a deep blue solution when dissolved in water. It is used as an antidote for cyanide poisoning and a bacteriological stain.