Define turn over number .

Explanation

**Step-by-Step Solution:**

1. **Definition of Turnover Number**: The turnover number (often represented as Kcat) is a measure used in enzymology to describe the efficiency of an enzyme. It indicates how many substrate molecules are converted into product by a single active site of the enzyme in a given unit of time.

2. **Context of Turnover Number**: The turnover number is specifically defined for enzymes, which are biological catalysts that speed up chemical reactions in living organisms.

3. **Components of the Definition**:
- **Substrate**: The molecule upon which an enzyme acts.
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