Convex Hull

Convex : A subset S of the plane is called convex if and only if for every pair of points p,q in S, the line segment pq is completely contained in S.

Convex Hull : The convex hull CH(S) of a set S is the smallest convex set that contains S.

http://www.seas.upenn.edu:8080/~cse121/lecture/lec03/index.htm