noun
hamster wheel; exercise wheel
A rotating wheel for small pets such as hamsters to run inside. Common in pet shops and home cages.
ハムスターが回し車で走っている。
The hamster is running on the wheel.
Compound of 回し (mawashi, 'turning') + 車 (kuruma, 'wheel'). The exact origin is uncertain, but it is a straightforward descriptive term.