Definition of: acceleration

(ak·sel′ə·rā′shən) noun
1. The act of accelerating, or the process of being accelerated; a quickening, as of progress, action, functional activity, etc.
2. The rate at which the speed of a body increases.
3. Physics The rate at which the velocity of a body increases per unit of time: used also of decrease of velocity, which is expressed as a negative acceleration.