A group action of a topological group on a topological space is said to be a proper group action if the mapping

is a proper map, i.e., inverses of compact sets are compact.

A proper action must have compact isotropy groups at all points of .

A group action of a topological group on a topological space is said to be a proper group action if the mapping

is a proper map, i.e., inverses of compact sets are compact.

A proper action must have compact isotropy groups at all points of .

Weisstein, Eric W. "Proper Group Action."
From *MathWorld*--A Wolfram Web Resource. https://mathworld.wolfram.com/ProperGroupAction.html