The half-life of knowledge or half-life of facts is the amount of time that has to elapse before half of the knowledge or facts in a particular area is superseded or shown to be untrue. These coined terms belong to the field of quantitative analysis of science known as scientometrics.
These ideas of half-life applied to different fields differ from the concept of half-life in physics in that there is no guarantee that the knowledge or facts in areas of study are declining exponentially. It is unclear that there is any way to establish what constitutes "knowledge" in a particular area, as opposed to mere opinion or theory.
It has also been used in Christian missiology to increase the effectiveness of their teachings.