In thermodynamics, work is one of the three principal classes of thermodynamic process by which a thermodynamic system can interact with its surroundings in energy transfer. Thermodynamic work occurs by a mechanism through which the system can spontaneously exert externally measurable macroscopic forces on its surroundings. In the surroundings, such mechanical work can be made to lift a weight. The remaining two principal classes of thermodynamic process are heat and transfer of matter.

These forces and external effects may be electromagnetic, gravitational, or mechanical (such as pressure-volume) variables. For thermodynamic work, appropriately chosen externally measured quantities are exactly matched by values of or contributions to changes in macroscopic internal state variables of the system, which always occur in conjugate pairs, for example pressure and volume or magnetic flux density and magnetization.

By an external system that lies in the surroundings (which is not necessarily a thermodynamic system as strictly defined by the usual thermodynamic state variables), otherwise than by heat or transfer of matter, work can be said to be done on a thermodynamic system. Part of such surroundings-defined work can have a mechanism just as for system-defined thermodynamic work done by the system, while the rest of such surroundings-defined work appears, to the thermodynamic system, not as a negative amount of thermodynamic work done by it, but, rather, as heat transferred to it. The paddle stirring experiments of Joule provide an example, illustrating the concept of isochoric (or constant volume) mechanical work, in this case sometimes called shaft work. Such work is not thermodynamic work as defined here, because it acts through friction, within, and on the surface of, the thermodynamic system, and does not act through macroscopic forces that the system can spontaneously exert on its surroundings, describable by its state variables. Surroundings-defined work can also be non-mechanical. An example is Joule heating, because it occurs through friction as the electric current passes through the thermodynamic system. When it is done isochorically, and no matter is transferred, such an energy transfer is regarded as a heat transfer into the system of interest.

In the International System of Units (SI), work is measured in joules (symbol: J). The rate at which work is performed is power, measured as joules per second and denoted with the unit watt (W).