A musical term, denoting the use of notes outside of the diatonic ("do-re-mi") scale of the key. A practice as old as music itself, and what differentiates one era from another - the 20th Century is the culmination of centuries of adding newer and newer ways to incorporate those "outside" notes. It's part of what differentiates Wagner from J.S. Bach, and Schönberg from Wagner. Or what differentiates Anthony Braxton from Jackie McLean from Charlie Parker.