A simple polygon that is not convex is called concave, non-convex or reentrant. A concave polygon will always have at least one reflex interior angle —that is, an angle with a measure that is between 180 degrees and 360 degrees exclusive.

A concave polygon is a polygon in which at least one interior angle is a reflex angle (angle greater than 180° but less than 360°). A concave polygon has at least 4 sides. It means that a triangle is always a convex polygon. The diagonals in a concave polygon may lie partly or fully outside the polygon.

Concave polygons are those polygons in which at least one interior angle is a reflex angle and it points inwards. They have a minimum of 4 sides and a few of the diagonals in a concave polygon may lie partly or fully outside it. All concave polygons are irregular because the interior angles are not equal.

A Concave polygon is a polygon that has at least one interior angle greater than 180 degrees. It must have at least four sides. The shape of the concave polygon is usually irregular. A concave polygon is a polygon which is not convex. This polygon is just the opposite of a convex polygon.