a poisonous trivalent and pentavalent solid element that commonly occurs in a brittle, metallic, steel-gray, crystalline form and is used especially in wood preservatives, alloys, and semiconductors
arsenic trioxide
noun(ar-se-nic)
a poisonous trioxide As2O3 or As4O6 of arsenic used especially as an insecticide or weed killer —called also arsenic trioxide