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 sigt = (8 pi e4)/(3 c4me2)

e is the charge on the electron, c is the speed of light and me is the mass of the electron

sigt = 6.6524-29m2