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Acceleration is change in velocity.

You will often hear people use acceleration to mean getting faster, and deceleration to mean slowing down. We use the accelerator pedal on a car to go faster, and the brake in order to decelerate. The mathematical term is a little more precise than this common usage because it includes the idea of the direction of acceleration. This may be the same as the way you are travelling - in which case you will go faster - or it may be in the opposite (negative) direction - in which case you will go slower. The direction of acceleration does not even need to be in line with the direction of motion. Think of the Moon. In what direction is it accelerating? In which direction is it moving?

Acceleration a is the derivative of velocity with respect to time: a = [dv/dt] .
Also if r is position, we have a = v [dv/dr] , because a = [dv/dt] = [dr/dt] [dv/dr] = v [dv/dr] .
Further, since v = [dr/dt] , we have a = [dv/dt] = [d/dt] [dr/dt] = [(d² r )/(dt²)] .
Acceleration has dimensions LT⁻², and SI unit ms⁻².