Inside vs. Outside
One definition of whether a point is inside a region is the Jordan Curve Theorem. Essentially, it says that a point is inside a polygon if, for any ray from this point, there is an odd number of crossings of the ray with the polygon's edges (Figure 1). This definition means that some areas which are enclosed by a polygon are not considered inside. The center pentagonal area inside a star is classified as outside because any ray from this area will always intersect an even number of edges.

Point in Polygon Strategies: Determining if a point lies within, or outside a polygon