ar·se·nic 
(ärsə-nĭk)
(ärsə-nĭk)n.
1. Symbol As 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. See Periodic Table.
2. Arsenic trioxide.
adj. ar·sen·ic (är-sĕnĭk)
Of or containing arsenic, especially with valence 5.
[Middle English arsenik, from Old French, from Latin arsenicum, from Greek arsenikon, yellow orpiment, alteration of Syriac zarnīkā, from Middle Persian *zarnīk, from Old Iranian *zarna-, golden; see ghel-2 in the Appendix of Indo-European roots.]