How fast velocity changes. It happens when something speeds up, slows down, or changes direction.
Usually measured as m/s2 ("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/s2