Define mean free path .

Explanation

**Step-by-Step Solution:**

1. **Definition of Mean Free Path**:
The mean free path is defined as the average distance a molecule can travel in a gas before colliding with another molecule.

2. **Understanding the Concept**:
When a molecule moves through a gas, it does not travel indefinitely in a straight line. Instead, it will collide with other molecules, which alters its direction and energy. The mean free path quantifies this behavior.

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