a flatted note, especially the third or the seventh degree of the scale, recurring frequently in blues and jazz as a characteristic feature.
Origin of blue note
First recorded in 1925–30
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2019
British Dictionary definitions for blue note
jazz a flattened third or seventh, used frequently in the blues
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012