"Bonus stages" refer to special levels or sections in video games that provide an extra challenge or reward, usually outside of the main gameplay. Full definition
There is also a couple of neat bonus stages in - between matches. (levelsmack.com)
In addition to the main story stages, there are also a handful of bonus stages that are unlocked as you get perfect scores on the higher difficulty levels. (zeboyd.com)
You can also enjoy bonus stages where the aim is to break as many blocks as possible within the time limit. (superluigibros.com)