What do you understand by the term nuclear fusion ?

Explanation

**Step-by-Step Solution:**

1. **Definition of Nuclear Fusion**:
- Nuclear fusion is defined as the process in which two or more atomic nuclei combine to form a single, heavier nucleus.

2. **Conditions for Fusion**:
- This process occurs under extreme conditions, specifically at very high temperatures and pressures. These conditions are necessary to overcome the electrostatic repulsion between the positively charged nuclei.

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