Tidal heating is heating from friction due to movements within a body due to tidal forces. The process is also called tidal working or tidal flexing. Tidal heating is one of the sources of heat in a body undergoing tidal force, for example, providing the heat to allow liquid water to exist in some solar system moons even though they are at sufficient distance that sunlight would provide insufficient heat.
Such tidal forces and heating occur when a body is not tidally locked with a body it is orbiting, resulting in different parts of it being close to the other body over time. They also occur when the when an orbit is eccentric, a condition that orbital resonances can drive, which gives some moons of Jupiter and Saturn the ability to harbor subsurface liquid water.