Definition:Region

Region in the Plane
The usual usage of region is in the real number plane or complex plane.

Interior
The boundary of a region separates its interior from the exterior.

The interior consists of the points of the plane which are the elements of the region.

Such points are called interior points of the region.

It is "usual" that the interior is the "smaller bit" which is visually apparently on the inside as it appears on the page or screen, but this is of course not necessarily the case.

Also see the definition of interior and boundary from a topological perspective.

Bounded
A region in the the plane is bounded if there is a circle in the plane which encloses it.

Also see the definition of bounded in the context of a metric space.

Also see

 * Definition:Open Region