By radioactive decay law. The time in which the final amount becomes half of its initial value is called half-life $(T)$ of that radioactive element.
Relation between half-life and disintegration (decay) constant is given
$T=\frac{0.6931}{\lambda}$$\Rightarrow T= \frac{\log _{e} 2}{\lambda}$
So, $\lambda T=\log _{e} 2$