What is the S.I. unit of mass ? How is it defined?

Explanation

### Step-by-Step Solution:

1. **Identify the S.I. Unit of Mass**:
- The S.I. (International System of Units) unit of mass is **kilogram**, abbreviated as **kg**.

2. **Define the Kilogram**:
- The kilogram is defined as the mass of a specific international prototype. This prototype is a physical object made of a platinum-iridium alloy, which is stored in a secure location in France.

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