An Etymological Dictionary of Astronomy and Astrophysics

A force that comes into play after a system is perturbed away from the equilibrium,
tending to bring the system back the equilibrium position. For example, when a pendulum is displaced from its equilibrium position, it is subject to a restoring force due to gravity that will accelerate it back toward the equilibrium position. The restoring force combined with the pendulum’s mass causes it to oscillate about the equilibrium position.

See also: → restore; → force.