Acceleration

[Sarah]

The act of accelerating, or the state of being accelerated;increase of motion or action; as, a falling body moves toward theearth with an acceleration of velocity; -- opposed to retardation.A period of social improvement, or of intellectual advancement,contains within itself a principle of acceleration. I. Taylor.(Astr.

[Sarah]

Defined as the change in velocity over a period of time. Acceleration is a vector quantity and therefore is stated as a quantity with a direction.
a = (v-u)/t (where a, v and u are vectors)
Where (using SI units):
a = acceleration of the body in metres per second per second (ms^-2, or metres per second squared)
v = final velocity of the body in metres per second (ms^-1)
u = initial velocity of the body in metres per second (ms^-1)
t = time period between the initial and final velocity, in seconds (s)
An accelerating body can also be decelerating (ie, negative acceleration) or be at rest. Also, instantaneous acceleration is a bit different.

[Sarah]

The process of getting high on speed

[Tina]

the increase in somethings speed, or its ability to go faster:

[Sarah]

A vehicles capacity to gain speed within a short time.