The acceleration is the action and the effect to accelerate (increase speed). This term can also denote the vector magnitude expressed by this increase in speed over a unit of time (meter per second squared according to his unit in the International System).
The acceleration can be negative; in these cases, greatness expresses a decrease of speed versus time.
It is important to distinguish the speed (which indicates how to change the position of one body with respect to time) and acceleration (which indicates how this same speed varies). Acceleration refers to how to change the speed and not how is the speed: a body that moves at high speed may have a very low acceleration.
There are several types of acceleration. The tangential acceleration is that which relates the variation of the speed over time. On the other hand, normal or centripetal acceleration associated changes of the direction over time.
In turn, the average acceleration to calculate the average change in speed over a given time interval.
The acceleration can be from different directions. When the body begins to reduce its speed, its acceleration moves in the direction opposite to its motion. On the other hand, if a body increases its speed (i.e., it accelerates), acceleration keeps the same direction as the speed.
Finally, we will retain that acceleration increases not only the speed of an engine, but also the pace of an organ: "green tea accelerated the beating of the heart.