arsenic trioxide

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A poisonous, white amorphous powder, As₂O₃, used in insecticides, rat poisons, weed killers, pigments, and preservatives for hides and wood.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun An industrially important oxide of arsenic that occurs as the mineral claudetite; formula As2O3 (though in liquid, gaseous and some solid forms it has molecules of the form As4O6).

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a white powdered poisonous trioxide of arsenic; used in manufacturing glass and as a pesticide (rat poison) and weed killer