The Collaborative International Dictionary of English v.0.48:

Aria \A"ri*a\, n. [It., fr. L. a["e]r. See Air.] (Mus.)
   An air or song; a melody; a tune.
   [1913 Webster]

   Note: The Italian term is now mostly used for the more
         elaborate accompanied melodies sung by a single voice,
         in operas, oratorios, cantatas, anthems, etc., and not
         so much for simple airs or tunes.
         [1913 Webster]

WordNet (r) 3.0 (2006):

aria
    n 1: an elaborate song for solo voice