Tidal locking
also called synchronous rotation
When a world's spin matches its orbit so the same face always points at its partner, the way the Moon always shows Earth one side.
A tidally locked world turns exactly once per orbit, so it keeps the same face toward whatever it circles. Our Moon does this with Earth: we only ever see one side of it from here. The lock builds up over millions of years, as gravity raises a slight bulge on a body and slowly drags its spin into step with its orbit.
For a planet close to its star, tidal locking can mean one hemisphere in permanent daylight and the other in endless night, with a ring of perpetual sunset in between. It is especially common for worlds in the habitable zones of small red dwarf stars, which sit close in. gravityfinder flags a world as likely locked from its orbit and its star, and never invents a length of day the data cannot support.
Related terms: Habitable zone · Surface gravity