In physics or physical science, acceleration (symbol: a) is defined as the rate of change (or derivative with respect to time) of velocity.
To accelerate an object is to change its velocity, which is accomplished by altering either its speed or direction (like in case of uniform circular motion) in relation to time.
In this strict mathematical sense, acceleration can have positive and negative vals (deceleration).
Any time that the sign (+ or -) of the acceleration is the same as the sign of the velocity, the object will speed up.
If the signs are opposite, the object will slow down.
Acceleration is a vector defined by properties of magnitude (size or measurability) and direction.
When either velocity or direction are changed, there is acceleration (or deceleration).