Hammer units, often abbreviated HUs, are the unit of measurement used by Source Engine games. It is also used to judge the speed of objects and players in-game, mostly for competitive play. 1 Hammer unit is 1.905 cm, and is the minimum distance the Source Engine can measure.
Many weapons such as the Stickybomb Launcher rely upon calculating Hammer Units for their projectiles' Damage rates and other effects.
A number of Achievements, such as "Project Excelsior" for the Soldier and "Operation High Dive" for the Demoman (which both use the Base Jumper's parachute) are reliant upon Hammer Units for calculation.