a very poisonous, white, crystalline compound, HgCl, used in photography and as an insecticide, antiseptic, etc.
See mercuric chloride in American Heritage Dictionary 4
noun
A poisonous white crystalline compound, HgCl2, used as an antiseptic and a disinfectant, in insecticides, preservatives, and batteries, and in metallurgy and photography. Also called corrosive sublimate.