...from a google search
Velocity - the force in which a note is pressed or played (attacked) and released. (i.e., the harder the note is pressed, the louder it will sound.) The range of velocities is 0-127. Some keyboards feature 'velocity sensitive keys' and some drum machines and percussion controllers may feature 'velocity sensitive pads'. Velocity on wind controllers may be controlled by the amount of air stream produced into the instrument.