Hello...I am having trouble understanding what gamma invariant is and was wondering if someone could help explain it to me.

My professor provides me notes and it says:

"A set x is said to be gamma invariant (where again gamma is a circle with center O) if the gamma inverse of each point on x is also on x."

This is just a really confusing definition for me and if anyone can explain it in plain ole' English I would be much obliged.

Thank you!