blue note

Short definition from Wiktionary

Noun

1. 1
   The flatted 5th scale degree in the blues scale.
2. 2
   Any of the notes that produce the characteristic clashes with the underlying harmony associated with blues music. The flatted third, flatted fifth and flatted seventh can all be considered blue notes when played over major or dominant chord progressions, while only the flatted fifth will clash with minor chord progressions.