HDR
HDR or High Dynamic Range is used as the name for a feature introduced into the Source engine with Half-Life 2: Lost Coast and Day of Defeat: Source.
For the player, Valve's "dynamic tonemapping" simulates the way that a human eye (or a camera with auto-exposure) gradually adjusts to compensate for changes in illumination levels, such as when entering a dark room from a daylit street or vice versa. As well as exposure-shifting to suit the average light level of the scene, the HDR feature also depicts burn-out (flat white with bloom halo) in over-bright areas, and flat shadows (black) in over-dark areas of the scene.
For example, standing in a street in bright daylight, the window of a house appears black - you cannot see any details of the room behind the window. On entering the house the interior still appears black until your eyes gradually adjust to the lower light level, and then you can see the furniture, etc. Now looking at the small window to the street from inside the house, the illumination outside is far too bright for your eyes tuned to the interior light level, so you just see a slab of blooming bright white light instead of the street or sky beyond.
The advantage of this 'LDR' view of an 'HDR' scene is that the correctly-exposed parts of the image are rendered in full detail; using all 8 bits per RGBA channel to describe the fine gradations of local colour and tone on the Player's screen.