Exponential decay
From Conservapedia
Exponential decay is the decay of a physical property whose value goes as
where 
is the value at time 
and 
expresses the rate of exponential decay. The quantity reaches one-half of its original value after a time interval equal to ln 2, which is called its half-life.
Examples
Examples of exponential decay include the radioactive decay of radioisotopes, the attenuation of sound or light through an absorber, and the decay of the magnetic fields of the planets.[1]
