noun
: either of two iodides of mercury:
a. : a yellow amorphous powder Hg 2 I 2 that turns greenish on exposure to light — called also mercurous iodide
b. : a red crystalline poisonous salt HgI 2 that changes to a yellow crystalline modification when heated above 126° C and that is used chiefly in medicine — called also mercuric iodide