Define half-life of a radioactive substance.

Explanation

**Step-by-Step Solution:**

1. **Definition of Half-Life**: The half-life of a radioactive substance is defined as the time interval required for the quantity (mass or number of atoms) of the substance to reduce to half of its initial value.

2. **Understanding the Concept**: When a radioactive substance undergoes decay, it transforms into other elements or isotopes over time. The half-life is a measure of how quickly this decay occurs.

3. **Mathematical Representation**: The half-life (T_half) is mathematically related to the decay constant (k) of the substance. The relationship is given by the formula:
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