How fast velocity changes.

Usually measured as m/s^{2} ("meters per second squared").

Example: going from 5 m/s (5 meters per second) to 6 m/s in exactly one second is an acceleration of 1 meter per second **per second**.

And two lots of "per second" becomes "per second squared".

So the acceleration is 1 m/s^{2}

See: Velocity