What is meant by the term inertia?

Explanation

**Step-by-Step Solution:**

1. **Definition of Inertia**: Inertia is defined as the property of matter that causes it to resist changes in its state of motion.

2. **Opposition to Change**: This means that if an object is at rest, it will stay at rest unless acted upon by an external force. Similarly, if an object is in motion, it will continue to move at a constant velocity in a straight line unless acted upon by an external force.

3. **External Force**: The term "external force" refers to any force that is applied from outside the object. This force is necessary to overcome the inertia of the object and change its state of motion.

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