Post-FX is something borrowed from film and print. It's the type of thing you would do after a render is complete. With real time stuff you are technically doing post-fx on a rendered frame in the same way but since its in real time the post-fx is a real time system that is designed to be tweaked and edited. It's more than…
isnt the whole scene post processed nowardays anyway since the renderer only creates them passes(diffuse, specular, normal,depth) and the rest like lighting and shading = one giant horny pixelshader over the screen buffer one does call that deffered shading
Screen space has other connotations I think. usually id associate screen space with post effects that simulate something like AO. its not raytraced rather its screen space. it is still however a post process. an effect handled once the render is complete. Our programmer used to call them passes. eg a bloom and SSAO pass.