The B-flat dominant seventh flat five chord (abbreviated "Bb7(b5) chord") is a four-note chord consisting of the notes B♭, D, F♭, and A♭. The chord is formed by lowering the fifth of a B-flat dominant seventh chord a half step.
B-flat dominant seventh flat five chord
B♭ D F♭ A♭
1 3 ♭5 ♭7
tetrad (four-note chord)