Define mass defect.

Explanation

### Step-by-Step Solution:

1. **Understanding Atomic Structure**:
- Atoms are made up of three types of subatomic particles: protons, neutrons, and electrons. Protons and neutrons are collectively known as nucleons, while electrons revolve around the nucleus.

2. **Defining Mass Defect**:
- The mass defect refers to the difference between the total mass of the individual nucleons (protons and neutrons) when they are free and the actual mass of the nucleus when they are bound together.

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