1. (Science: chemistry) 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 colour 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 arsenticum of the ancients. The element and its compounds are active poisons. Specific gravity from 5.7 to 5.9. Atomic weight. Symbol as.