mercurous chloride, HgCl, a white, tasteless powder that darkens on exposure to light: used in standard electrode cells and as a fungicide, insecticide, etc.
See calomel in American Heritage Dictionary 4
(kălˈə-mĕlˌ, -məl)
noun
A colorless, white or brown tasteless compound, Hg2Cl2, used as a purgative and insecticide. Also called mercurous chloride.