Velocity

Velocity is a rate of change over time.

When playing a keyboard the velocity is how fast the note is played, not how hard.

In MIDI, velocity is a value represented by one of the data bytes of the channel voice note on (and off) message. Many synths and other MIDI devices allow the user to route the velocity information to parameters other than notes, such as filters,LFO's and modulation.

Formula
V = Distance/Time