Define atomic mass unit.

Explanation

### Step-by-Step Solution:

1. **Understanding Atomic Mass Unit (AMU)**:
- The atomic mass unit is a standard unit of mass that quantifies mass on an atomic or molecular scale. It is particularly useful for measuring the mass of atoms and subatomic particles.

2. **Definition of Atomic Mass Unit**:
- The atomic mass unit is defined as one twelfth (1/12) of the mass of an unexcited atom of carbon-12. This specific isotope of carbon is chosen as a reference point for defining the atomic mass unit because it is stable and abundant.

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