What is the meaning of a.m.u. (atomic mass unit) ?

Explanation

### Step-by-Step Solution:

1. **Definition of Atomic Mass Unit (AMU)**:
The atomic mass unit (AMU) is a standard unit of mass that is used to express atomic and molecular weights. It is particularly useful for measuring the mass of very small entities such as atoms and subatomic particles.

2. **Relation to Carbon-12**:
The AMU is defined based on the mass of a specific isotope of carbon, known as carbon-12. Specifically, 1 AMU is defined as one-twelfth (1/12) of the mass of an unexcited carbon-12 atom.

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