The Greek letter that usually denotes the Lorentz factor is gamma, written γ.
The Lorentz factor is defined as γ=1/√(1−v²/c²), where v is relative speed and c is the speed of light. It measures how strongly special-relativistic effects appear as an object’s speed approaches c. At rest, γ equals 1; at higher speeds, it grows rapidly.
Gamma appears in the Lorentz transformations and in equations for time dilation, length contraction, relativistic momentum and total energy. For example, a moving clock’s elapsed time is related to proper time through the Lorentz factor.
The symbol is connected with the work of Dutch physicist Hendrik Lorentz, whose name also gives the factor its title. Beta, β, commonly represents the speed ratio v/c, while gamma is the factor calculated from that ratio. At 86.6% of light speed, γ is exactly 2.