Write down the definition and formula of half-life of a radioactive substance.

(N/A) Definition: The half-life $(T_{1/2})$ of a radioactive substance is defined as the time interval during which the number of radioactive nuclei in a given sample reduces to half of its initial value.
Formula: The half-life is related to the decay constant $(\lambda)$ by the following formula:
$T_{1/2} = \frac{\ln(2)}{\lambda} \approx \frac{0.693}{\lambda}$
Where:
$T_{1/2}$ is the half-life,
$\lambda$ is the radioactive decay constant.