Half-Life High Definition Content
The Half-Life High Definition Content was the content included with the Blue Shift installation of Half-Life (which allowed for various custom content packs, such as ones made by users). With the release of Blue Shift on Steam, the content was added as an on-demand update (although it was never mentioned).
The High Definition Content originated in the Dreamcast version of Half-Life. The poly-count of many of the models was increased, though not to the same extent as the current High Definition Content.
The High Definition Pack on Steam contains only the animations required for Half-Life, Blue Shift and Opposing Force, making many mods that use skinned Half-Life models unplayable.