Define Reduction in terms of Hydrogen and Oxygen.

Explanation

### Step-by-Step Solution:

1. **Definition of Reduction**:
- Reduction can be defined as a chemical process where a substance gains hydrogen or loses oxygen.

2. **Reduction in Terms of Hydrogen**:
- When a substance gains hydrogen, it is said to be reduced.
- For example, consider the reaction of nitrogen with hydrogen to form ammonia:
\[ N_2 + 3H_2 \rightarrow 2NH_3 \]
- In this reaction, nitrogen (N₂) gains hydrogen (H₂) to form ammonia (NH₃). Thus, nitrogen is reduced.

3. **Reduction in Terms of Oxygen**:
- Alternatively, reduction can also be defined as the loss of oxygen from a substance.
- For instance, consider the reaction of copper oxide (CuO) with hydrogen:
\[ CuO + H_2 \rightarrow Cu + H_2O \]
- In this reaction, copper oxide loses oxygen and is converted to copper (Cu). Therefore, copper oxide is reduced.

4. **Summary**:
- Reduction can be summarized as:
- **Gain of Hydrogen**: A substance is reduced when it gains hydrogen.
- **Loss of Oxygen**: A substance is reduced when it loses oxygen.