The Collaborative International Dictionary of English v.0.48:

Arpeggio \Ar*peg"gio\, n. [It., fr. arpeggiare to play on the
   harp, fr. arpa harp.] (Mus.)
   The production of the tones of a chord in rapid succession,
   as in playing the harp, and not simultaneously; a strain thus
   played.
   [1913 Webster] Arpent

WordNet (r) 3.0 (2006):

arpeggio
    n 1: a chord whose notes are played in rapid succession rather
         than simultaneously