1. Not open or affording passage or access
2. Not open to the general public
3. Shut down
4. Blocked against entry; SYN. closed in.
5. Not having an open mind; SYN. unsympathetic.
6. Requiring union membership
7. (Mathematics) Of a curve or surface; having no end points or boundary curves; of a set; having members that can be produced by a specific operation on other members of the same set; of an interval; containing both its endpoints.
8. Used especially of mouth or eyes; SYN. shut.
9. With shutters closed.
10. (Of the wings of birds and insects) Closed together.
In mathematics, descriptive of a set of data for which an operation (such as addition or multiplication) done on any members of the set gives a result that is also a member of the set.
For example, the set of even numbers is closed with respect to addition, because two even numbers added to each other always give another even number.