This is the ratio of the scatterd flux divided by
the incident flux for a bunch of photons hitting an electron. It has units of

area and classically represents
the

size that the

electron appears to be to the

photon.

It is given by
sig_{t} = (8 pi e^{4})/(3 c^{4}m_{e}^{2})

e is the charge on the electron, c is the speed of light and
m_{e} is the mass of the electron

sig_{t} = 6.6524^{-29}m^{2}