In GIS a convex hull (also convex polygon or convex envelope) takes the outermost nodes of a vector shape (points, lines or polygons) and creates a polygon with the smallest area that encompasses all features and has no concave angles. In a convex polygon a straight line drawn between any two points inside the polygon is completely contained within the polygon. Visually, a convex hull is often explained by imagining the shape that a rubber band would take around a group of objects. Creating a convex hull is standard component of the geometry toolset of GIS applications.
This is in contrast to a concave-hull, which does allow concave angles in the envelope. Some graphic examples are presented at http://ubicomp.algoritmi.uminho.pt/local/concavehull.html