OuterPolygon

OuterPolygon[poly] gives the outer polygon of the polygon poly.

Examples

Get the outer polygon of a polygon with a hole:

OuterPolygon[Polygon[{{{0,0},{1,0},{1,1},{0,1}},{{0.2,0.2},{0.8,0.2},{0.8,0.8},{0.2,0.8}}}]]
(* Polygon[{{0,0},{1,0},{1,1},{0,1}}] *)

Please visit the official Wolfram Language Reference for more details.