Tidal locking

Tidal locking occurs when two celestial bodies orbiting each other reach a state where one of them experiences no net change in its rotation rate during a complete orbit. In synchronous rotation, a tidally locked body takes as much time to rotate around its own axis as it does to revolve around its partner. Typically, only the satellite becomes tidally locked to the larger body. Yet, if the mass difference and distance between the two bodies are relatively small, both may be tidally locked to each other.

Examples

The planet of Verces is tidally-locked with the sun of the Pact Worlds system, so that one side of Verces (known as Fullbright) is always facing the sun while the other (known as Darkside) is in perpetual darkness.