One of the great things about being a grad student at Georgia State is that I have access to the OED -- the Oxford English Dictionary!
1. The quality or fact of being dissonant; an inharmonious or harsh sound or combination of sounds; = DISCORD 3a, 4. spec. in Music, A combination of tones causing beats (cf. BEAT n.1 8), and thus producing a harsh effect; also, a note which in combination with others produces this effect.