Define decay constant

Explanation

### Step-by-Step Solution:

1. **Understanding Decay**: The decay of a radioactive substance refers to the process by which unstable nuclei lose energy by emitting radiation. This process occurs randomly over time.

2. **Rate of Decay**: The rate of decay of a radioactive substance is defined as the number of nuclei that decay per unit time. This rate is not constant; it depends on the number of undecayed nuclei present at any given moment.

3. **Proportionality**: The rate of decay is proportional to the number of undecayed nuclei (N) present at that instant. Mathematically, this can be expressed as:
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