A probabilistic rate of decay where the quantity of something is reduced by half. Used most often in physics to describe the exponential decay of radioactive elements.
Coined by physicist Ernest Rutherford in 1907 when he discovered the phenomenon of the rate of radioactive decay to determining the ages of rocks. (source)
We experience "decay" in our everyday lives, whether in habits that have gone stale, knowledge that we acquired through formal education but now struggle to recall, or skills that dwindled without practice. Reminding ourselves that all of these things have their own "half-life" helps us to not take it personally — that often our assets exhibit a 'use-it-or-lose-it' quality that must be cultivated and grown.