Screen Space Ambient Occlusion (SSAO)
Ambient Occlusion (AO) is a type of lighting technique applied in-game to create more realistic lighting. AO simulates soft shadows created by two surfaces that are close together. Ambient occlusion in games is normally accomplished through Screen Space Ambient Occlusion, which is a simplified way to fake realistic AO with shaders in a process fast enough for real-time rendering. SSAO is a DirectX 9.0c shader, so it will only be available on fairly modern video cards.
Ambient occlusion is currently only available for NVidia cards using the NVidia drivers. It is very resource-intensive and will cause a significant reduction in framerate on older video cards.
SSAO causes several rendering bugs which are common to nearly all games which offer SSAO.
Enabling ambient occlusion
Ambient occlusion cannot be controlled through the video options within a Source game. AO can be turned on with NVidia cards using the NVidia drivers.