The rate of decrease in velocity is called deceleration or retardation. It is the opposite of acceleration, which is the rate of increase in velocity.

Here's a breakdown:

* Velocity: A vector quantity describing both speed and direction.

* Acceleration: The rate of change of velocity over time. It can be positive (speeding up), negative (slowing down), or zero (constant velocity).

* Deceleration: A specific type of acceleration where the velocity is decreasing, meaning the object is slowing down. It is often referred to as negative acceleration.

Important Note: While the term "deceleration" is commonly used, it's technically a specific case of acceleration. The direction of acceleration determines whether an object is speeding up or slowing down.