A gas holder or gasholder, also known as a gasometer, is a large container in which natural gas or town gas is stored near atmospheric pressure at ambient temperatures. The volume of the container follows the quantity of stored gas, with pressure coming from the weight of a movable cap. Typical volumes for large gas holders are about 50,000 cubic metres (1,800,000 cu ft), with 60-metre (200 ft) diameter structures.

Gas holders now tend to be used for balancing purposes to ensure that gas pipes can be operated within a safe range of pressures, rather than for actually storing gas for later use. The anglicisation gasometer was adopted by William Murdoch, the inventor of gas lighting, in 1782, as the name for his gas holders.[3][4] Murdoch's associates objected that his gasometer was not a meter but a container, but the name was retained and came into general use. Gas holders were marked as gasometers on the large-scale maps issued by the British Ordnance Survey and term came to be used to label gas works, even though there may be several gas holders at any one gas works. However, the term gasometer is still discouraged for use in technical circles, where gas holder is preferred.[5]