In firearms, the chamber is the portion of the barrel or firing cylinder in which the cartridge is inserted before being fired. Rifles and pistols generally have a single chamber in their barrels, while revolvers have multiple chambers in their cylinders and no chamber in their barrel. Thus rifles and pistols can usually be fired even with a detached magazine, while a revolver cannot be fired with its cylinder swung out.
The act of chambering a cartridge means the insertion of a round into the chamber, either manually or through the action of the weapon, e.g., pump-action, lever-action, bolt action, or automatic action generally in anticipation of firing the weapon, without need to 'load' the weapon upon decision to use it (reducing the number of actions needed to discharge).
In firearms design or modification, "chambering" is fitting a weapon for a particular caliber or round, so a Colt Model 1911 is chambered for .45 ACP or .38 Super, or re-chambered for .38/.45 Clerke.