Power is a rate, the rate at which energy is expended or work done. The watt is defined as “the power which in one second gives rise to energy of 1 joule” (Resolution 2 of the CIPM, 1946, ratified by the Ninth CGPM in 1948). In mechanical terms, a power of 1 watt can, in 1 second, move a mass of 1 kilogram through a distance of 1 meter with such force that the kilogram's velocity at the end of the meter will be 1 meter per second greater than it was at the beginning. In an electric circuit, 1 watt is a current of 1 ampere at a pressure of 1 volt.

The watt's dimensions are (L²M)/T³ — joules per second, or in terms of base units,

The watt was introduced at the Second International Congress of Electricians (Paris, 1889), where it was defined as 10⁷ ergs per second. (The same conference introduced the joule, defining it as 10⁷ ergs.)

The watt is named for James Watt (1736-1819), inventor of the condensing steam engine.