Lorentz Factor 

It is defined by
${\gamma}=\frac{1}{\sqrt{1\frac{v^2}{c^2}}}$
where $v$ is the velocity of a moving frame relative to an inertial reference frame and $c$ is the speed of light.
See also Time Dilation, Length Contraction
Further reading: http://en.wikipedia.org/wiki/Lorentz_factor