From Jamey Aebersold's "Cantaloupe Island," Maiden Voyage, vol. 54.
This is called a bend!
It (normally) means exactly what it sounds and looks like: you start on the pitch, bend it down, and then return to the original pitch. More rarely, composers will notate bends to also mean you scoop up into the pitch.
Bends are really reserved for brass and woodwind players, so the fact that this is in a piano score suggests that it's probably an arrangement of a chart played by brass/wind players.
Edit: After hearing the example here, two things become clear:
- The notation here is for the latter intention mentioned above, where one is to scoop up into the note.
- It's played by piano, so the notation actually creates a bit of a grace-note E♭ that scoops up and resolves to F.
This example is from a c.1991 Hancock arrangement for a treble clef solo workbook. The articulation is indeed traditionally called a bend and does not appear in other charts of this standard tune. I’ve never seen it on a piano part but I would perform it as a kind of reverse mordant on the top note F. The middle finger bounces on the Eb while the fourth finger sustains the F, etc.
If playing right-hand synth, you may also bend by briefly flicking the pitch bender down during the note, having set the synth to top-note-only pitch bend mode.
I don’t know about brass but on sax the bend can be done by slightly dropping the tongue and jaw quickly during the note.