Define absolute refractive index.

Explanation

### Step-by-Step Solution:

1. **Understanding Refractive Index**:
The refractive index is a dimensionless number that describes how light propagates through a medium compared to a vacuum.

2. **Definition of Absolute Refractive Index**:
The absolute refractive index of a medium is defined as the ratio of the speed of light in a vacuum (denoted as 'C') to the speed of light in that medium (denoted as 'V').

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