Define the decay constant and state its $SI$ unit.

(N/A) The decay constant $(\lambda)$ of a radioactive substance is defined as the reciprocal of the time during which the number of atoms of the radioactive substance reduces to $1/e$ (approximately $36.8\%$) of its initial value.
Alternatively, it is the probability of decay per unit time for a radioactive nucleus.
Mathematically, it is given by the relation $dN/dt = -\lambda N$, where $dN/dt$ is the rate of decay and $N$ is the number of radioactive nuclei present at that instant.
The $SI$ unit of the decay constant is $\text{s}^{-1}$ (per second).