I believe the correct term would be a "polyhedron".
its self because a plane figure is a figure that is flat and has all strait sides.
A flat three sided figure is a triangle.
Add the areas of all the faces (Flat sides).
A square.
Square
A pentagon is a plane (flat) figure with five sides.
The length around a flat figure or the sum of its sides.
a polyhedron is any 3d figure with all flat polygons for sides(polygons have straight sides)
The ones the Aztecs build have 4 sides + 1 underside and 1 flat-top = 6 sides
The description given fits that of a triangle.
A hexadecagon is a polygon (a flat figure made of straight lines) with 16 sides.
If "colsed" is meant to be "closed", and if the figure is planar (flat), and if the sides of the figure do not cross, then it is a polygon. But answer to the question could include shapes such as stars (pentagrams) which are not polygons..