Let be a topological space and A be a subset of X, then a point , is said to be an exterior point of A, if there exist an open set U, such that
                                                            

            In other words, let A be a subset of a topological space X. A point  is said to be an exterior point of A if there exists an open set U containing such that.
 
Exterior of a Set:
            The set of all exterior points of A is said to be the exterior of A and is denoted by.

Remark:
            It may be noted that an exterior point of A is an interior point of.

Theorems:

• If A is a subset of a topological space X, then (1)  (2) .
• If A is a subset of a topological space X, then.
• In a topological space X, (1)  (2) .
• If A is a subset of a topological space X, then (1)  (2) .
• If A is a subset of a topological space X, then is the largest open subset of.