What is meant by mass defect?

Explanation

### Step-by-Step Solution:

1. **Definition of Mass Defect**:
- Mass defect refers to the loss of mass that occurs during a nuclear reaction. It is the difference between the mass of the individual nucleons (protons and neutrons) when they are free and the mass of the nucleus they form.

2. **Formation of a Nucleus**:
- Consider a nucleus denoted as X, which has an atomic number Z (number of protons) and a mass number A (total number of nucleons, which includes both protons and neutrons). The number of neutrons in the nucleus can be calculated as A - Z.

...