arsenic

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

• noun A highly poisonous metallic element having several allotropic forms of which the brittle, crystalline gray metallic form is the most common. The less stable yellow allotrope has a molecular structure, As₄. Arsenic and its compounds are used in insecticides, weed killers, semiconductor dopants, and various alloys. Atomic number 33; atomic weight 74.9216; valence 3, 5. Gray arsenic melts at 817°C (at 28 atm pressure), sublimes at 616°C, and has a specific gravity of 5.75. cross-reference: Periodic Table.
• noun Arsenic trioxide.
• adjective Of or containing arsenic, especially with valence 5.

from The Century Dictionary.

• noun A yellow mineral, called specifically yellow arsenic; the trisulphid of the element to which it has given its name; orpiment.
• noun Chemical symbol, As; atomic weight, 75. A chemical element having a grayish-white color, a metallic luster, and a specific gravity of 5.727.
• noun The popular name of arsenic trioxid (As₂O₃), the preparation of arsenic usually retailed in trade. See above.
• Containing arsenic; specifically, containing arsenic in smaller proportion than arsenious compounds. See arsenious.

from the GNU version of the Collaborative International Dictionary of English.

• adjective (Chem.) Pertaining to, or derived from, arsenic; -- said of those compounds of arsenic in which this element has its highest equivalence.
• noun (Chem.) One of the elements, a solid substance resembling a metal in its physical properties, but in its chemical relations ranking with the nonmetals. It is of a steel-gray color and brilliant luster, though usually dull from tarnish. It is very brittle, and sublimes at 356° Fahrenheit. It is sometimes found native, but usually combined with silver, cobalt, nickel, iron, antimony, or sulphur. Orpiment and realgar are two of its sulphur compounds, the first of which is the true arsenicum of the ancients. The element and its compounds are active poisons. Specific gravity from 5.7 to 5.9. Atomic weight 75. Symbol As.
• noun (Com.) Arsenious oxide or arsenious anhydride; -- called also arsenious acid, white arsenic, and ratsbane.

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

• noun A nonmetallic chemical element (symbol As) with an atomic number of 33.
• noun Arsenic trioxide.
• adjective Of, or containing arsenic with a valence of 5.

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
• 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