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An F set is a countable union of closed sets.

"May I ever so politely inquire what we can do for you on this closed set?"

The closed sets, as usual, are those whose complement is open.

That is, all of the points not belonging to a certain open sets forms a closed set.

This will not be the case with a general rotation in 3-space, which do form a closed set under composition.

And you told me it was a closed set, with no visitors allowed.

In some cases it is more convenient to use a base for the closed sets rather than the open ones.

Every open set is the union of closed sets.

Other equivalent definitions can be given, for example, in terms of closed sets and closure.

The only closed sets are the empty set and X.