In exponential decay (such as radioactive decay), there is a concept of "half life" $t_{1/2}$. It indicates a decay with "time". But if the x-axis is not "time" (such as for an exponential probability distribution), but works exactly same as a half life; is there any name for the analogous concept where there is such a $\Delta (X)$ interval that makes $f(x)$ half each time?
Bonus question: is it closely related to $1/\lambda$? is there a name for $1/\lambda$?
It is called the "decay constant."