Charcoal filtration is used to reduce or eliminate off flavors in spirits. Most common in neutral spirits like vodka, it can be used with most spirits. The process will also remove positive flavor components.
Lincoln County Process
The Lincoln County Process is a step used in producing almost all Tennessee whiskeys. The whiskey is filtered through, or steeped in, charcoal chips before going into the casks for aging. The process is named for Lincoln County, Tennessee, which was the location of Jack Daniel's distillery at the time of its establishment; subsequent redrawing of county lines means that none of the distilleries currently using the process are located in the county for which the process is named.