The amount of electric power for which a generating unit, generating station, or other electrical apparatus is rated either by the user or manufacturer. The term is also used for the total volume of natural gas that can flow through a pipeline over a given amount of time, considering such factors as compression and pipeline size.
There are various types of electricity capacity.:
Dependable Capacity: The systems's ability to carry the electric power for the time inrval and period specific, when related to the characteristics of the load to be supplied. Dependable capacity is determined by such factors as capability, operating power factor, weather, and portion of the load the station is to supply.
Installed (or Nameplate) Capacity: The total manufacturer-rated capacities of equipment such as turbines, generators, condensers, transformers, and other system components.
Peaking Capacity: The capacity of generating equipment intended for operation during the hours of highest daily, weekly or seasonal loads.
Purchased Capacity: The amount of energy and capacity available for purchase from outside the system
Reserve Capacity: Extra generating capacity available to meeet peak or abnormally high demands for power and to generate power during scheduled or unscheduled outages. Units available for service, but not maintained at operating temperature, are termed "cold." those units ready and avaiable for service, though not in actual operation, are termed "hot."

This definition is in context to Energy. See more contextual defintions for Capacity.