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