Velocity (MIDI)
How hard a key was struck, measured as how fast it travelled and sent as a single number inside MIDI's Note On message.
The keyboard times the gap between two contacts under each key and turns it into a value from 1 to 127, which rides along in the Note On message after the note number. What it means is up to the receiver: usually loudness, sometimes filter cutoff or which layer of a multisampled piano gets played. Velocity 0 is a special case meaning Note Off — a trick that saves bytes under running status and trips up everyone who writes their first MIDI parser.
Release velocity is in the spec too, but hardly any keyboards measure it. Because players and keybeds differ, most instruments let you pick a velocity curve. MIDI 2.0 raised the resolution to 16 bits.