Tidally locked planets and moons (or other tidally locked celestial bodies) are objects whose rate of rotation matches their orbital period around the object they orbit. As such, the same side of the orbiting body always faces the central object (though there may be some slight wiggle). Our own moon experiences such a tidal lock on Earth, and thus the same side of the moon is always facing the Earth while the dark side of the moon always faces away.

Planets tidally locked to a star will have one side of their surface experience perpetual daytime while the other experiences perpetual night. There may be extreme temperature differences between the opposite sides of the planets surface. Both air and sea currents will be impacted.

Worldbuilders wishing to generate believable ecosystems for tidally locked planets should consider how life may have evolved or adapted to live on one side of the planet or the other.