Deceleration is synonymous with slowing down or is related to the relative directions of velocity and acceleration. The meaning of the word in common language differs from its meaning in physics.
In everyday language, the word deceleration is typically used to signify the opposite of acceleration. When motion or speed increases, the body is said to accelerate. Conversely, when motion or speed decreases, the body is said to decelerate. Another way to describe deceleration is as a decrease in the magnitude of velocity.
However, in physics, acceleration is defined as a change in velocity when the motion is unidirectional. Acceleration means that acceleration and velocity point in the same direction, meaning both positive and negative velocities can be measured. For example, a positive velocity is described as an occasion where time increases and the velocity at which the object moves away from its starting point increases. Negative velocity is where time decreases and the velocity at which the object moves away from its starting point decreases.
Deceleration refers to when acceleration and velocity point in opposite directions. As with acceleration, deceleration may be described in terms of both positives and negatives. Negative velocity combined with positive acceleration is one type of deceleration, while a positive velocity with negative acceleration is another type.