arsenic

(noun) a white powdered poisonous trioxide of arsenic; used in manufacturing glass and as a pesticide (rat poison) and weed killer. Synonyms: arsenic trioxide, arsenous anhydride, arsenous oxide, white arsenic, ratsbane.

(noun) a very poisonous metallic element that has three allotropic forms; arsenic and arsenic compounds are used as herbicides and insecticides and various alloys; found in arsenopyrite and orpiment and realgar. Synonyms: As, atomic number 33.