blue note definition by Webster's New World
Webster's New World College Dictionary Copyright © 2010 by Wiley Publishing, Inc., Cleveland, Ohio. Used by arrangement with John Wiley & Sons, Inc.
Jazz a slightly flatted note, esp. the third or seventh note of a major scale
blue note definition by American Heritage Dictionary
A flatted note, especially the third or seventh note of a scale, in place of an expected major interval.
Origin: From its use in blues music.