Here's the breakdown:

* Acceleration is the rate of change of velocity. It can be positive (speeding up), negative (slowing down), or zero (constant velocity).

* Retardation is a term often used to describe *negative acceleration*, meaning the object is slowing down.

Key Point: Retardation is a specific case of acceleration, where the acceleration is in the opposite direction to the object's motion.

Example:

Imagine a car braking to a stop. Its velocity is decreasing, meaning it has negative acceleration, which is also called retardation.

So, while zero acceleration means no change in velocity, retardation means the object is slowing down, which implies a non-zero (negative) acceleration.