The<unk>  of a quantity, subject to exponential decay, is the time required for the quantity to decay to<unk>  of its initial value.  The concept originated in the study of radioactive decay, but applies to many other fields as well, including phenomena which are described by non-exponential decays.
The term<unk>  was coined in 1907, but it was always referred to as<unk>  period. It was not until the early 1950s that the word period was dropped from the name. 
The table at right shows the reduction of the quantity in terms of the number of<unk> s elapsed.
It can be shown that, for exponential decay, the<unk>  
t
1
/
2
{\displaystyle t_{1/2}}
 obeys this relation:
where
ln
⁡
(
2
)
{\displaystyle \ln(2)}
 is the natural logarithm of 2 (approximately 0.693), and
λ is the decay constant, a positive constant used to describe the rate of exponential decay.
The<unk>  is related to the mean<unk> time τ by the following relation:
Half-life
Fluency
Saturation 
Density
Molecular mass
Gas constant
Concentration
Time in therapeutic range