C 7alt, 2nd inversion (Bb in the bass)
Also written as C7alt/Bb.
Notes, from the bass up
Bb (bass)DbD#AbCE
The same notes as C 7alt (Bb, Db, D#, Ab, C and E), rearranged so Bb is the lowest note. Use it to move between chords with smaller jumps across the keyboard.