1. [n]  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
2. [n]  a white powdered poisonous trioxide of arsenic; used in manufacturing glass and as a pesticide and weed killer

ARSENIC is a 7 letter word that starts with A.

1. \Ar"se*nic\ ([aum]r"s[-e]*n[i^]k; 277), n. [L.
   arsenicum, Gr. 'arseniko`n, 'arreniko`n, yellow orpiment,
   perh. fr. 'arseniko`s or better Attic 'arreniko`s masculine,
   'a`rrhn male, on account of its strength, or fr. Per.
   zern[=i]kh: cf. F. arsenic.]
   1. (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[deg]
      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.
   2. (Com.) Arsenious oxide or arsenious anhydride; -- called
      also {arsenious acid}, {white arsenic}, and {ratsbane}.
   

2. \Ar*sen"ic\, a. (Chem.)
   Pertaining to, or derived from, arsenic; -- said of those
   compounds of arsenic in which this element has its highest
   equivalence; as, arsenic acid.