Definition
Exterior
The exterior of a closed shape is the region outside its boundary.
What it means
Imagine the boundary as a fence around a shape. Points beyond the fence are in the exterior. Points inside are in the interior, and points exactly on the fence belong to the boundary instead.
Remember it
Exterior means beyond the edge: outside the boundary, not sitting on it.
Example
Draw a triangle and mark point P well beyond one side. P is in the triangle’s exterior. A point drawn exactly on a side is on the boundary, so it is not an exterior point in the strict sense.
Watch for
In casual speech, outside may include an edge. The strict geometric exterior excludes every point on the boundary.
Common mix-up
An exterior angle is an angle outside a polygon; it is related, but it is not the whole exterior region.
Exact meaning
For a closed plane figure or region, the exterior is the set of points outside its boundary. In the strict geometric sense, boundary points are not part of the exterior.