In relativistic laser-plasma physics the relativistic similarity parameter S is a dimensionless parameter defined as S = n e a 0 n c r {\displaystyle S={\frac {n_{e}}{a_{0}n_{cr}}}},where n e {\displaystyle {n_{e}}} is the electron plasma density, n c r = m e ω 0 2 / 4 π e 2 {\displaystyle {n_{cr}=m_{e}\omega _{0}^{2}/4\pi e^{2}}} is the critical plasma density and a 0 = e A / m e c 2 {\displaystyle {a_{0}=eA/m_{e}c^{2}}} is the normalized vector potential. Wikipedia