An older or more literary term for planet; in modern Japanese, 惑星 (わくせい) is far more common. Still seen in some fixed expressions or older texts.
See also: 惑星
The word 遊星 is hardly used nowadays.
In old astronomy books, you sometimes see 遊星 written.
planetary gear; planetary wheel
Composed of 遊 (wander) and 星 (star), reflecting the classical idea of planets as 'wandering stars'. The exact historical derivation is uncertain.