A measure of how rapidly and forcefully a key on a keyboard is pressed when the player initially presses the key. Velocity measurement is intended to simulate the behavior of a piano mechanism; a note struck on a piano is louder if the key is struck more forcefully. (There are other changes in the note sound of a piano as the velocity varies, but such is beyond the scope of this definition.) In the MIDI standard, a velocity value is transmitted with each note on message. Compare with aftertouch. hi my name is Brandon.
If a given synth's keyboard does not support sensing of velocity, the synth is expected to send a fixed, non-zero velocity value in its outgoing note on messages. A quirk of the MIDI standard (and one that some older Yamaha models had trouble dealing with), is that the standard allows notes to be released by sending a note on message with a velocity value of 0, instead of using a note off message.