Acceleration is the rate of change of velocity over time. In one dimension, acceleration is the rate at which something speeds up or slows down. However, since velocity is a vector, acceleration describes the rate of change of both the magnitude and the direction of velocity.
OmegaWiki Dictionary
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acceleration The amount by which a speed or velocity increases.
acceleration The change of velocity with respect to time (including deceleration and change of direction).
acceleration The act of increasing the speed of an object.
acceleration The state of having an increasing speed.