Why is AO considered Post-Processing? Ambient Occlusion is a physical approximation to occlude ambient light. If ambient light is not considered post-processing, why is AO? Ambient occlusion has the same inputs as regular deferred lighting, plus kernels and a noise map, but essentially, it can be handled the same way. Is there a reason why it's considered post-processing and deferred lighting/shading isn't?
This is a potentially confusing re-use of terminology. Ambient Occlusion is not in itself a post processing effect. Screen Space Ambient Occlusion is a post processing effect using the depth buffer to approximate the effects produced by Ambient Occlusion.

Ambient Occlusion is a relatively expensive global method, while Screen Space Ambient Occlusion is the cheaper alternative for real time animation. Only the latter is considered post processing.