What is meant by the term retardation ? Name its S.I. unit.

Explanation

### Step-by-Step Solution:

1. **Definition of Retardation**:
- Retardation refers to the process where an object slows down or decreases its velocity over time. It is essentially the opposite of acceleration.

2. **Understanding Negative Acceleration**:
- In physics, when we talk about acceleration, we usually refer to an increase in velocity. However, when the velocity of an object decreases, we call this negative acceleration, which is synonymous with retardation.

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