Polygon

A polygon is a closed plane figure formed by a finite sequence of straight line segments joined end to end. Its segments meet only at their endpoints, and its boundary encloses a region.

Vertex (geometry): A point where two or more geometric lines, edges, or rays meet. Each polygon vertex joins two consecutive sides.
Polygonal chain: A sequence of line segments joined end to end, which may be open or closed. A polygon is the closed case of a polygonal chain.
Polygon triangulation: A partition of a polygon into triangles using noncrossing diagonals between its vertices. Triangulation supports area calculations and efficient geometric algorithms.
Convex polygon: A polygon in which every interior angle is less than 180° and every segment between points in the figure stays inside it. Convexity distinguishes polygons without inward dents from concave ones.
Line segment: A part of a line bounded by two endpoints. A finite chain of these straight pieces forms a polygon’s boundary.
Polygon interior: The bounded planar region enclosed by a polygon’s boundary. The closed boundary separates this region from the exterior.
Tessellation: A covering of a plane with shapes that leaves no gaps or overlaps. Polygonal tiles can fit together to cover a plane.
Concave polygon: A polygon with at least one interior angle greater than 180°. Its inward indentation contrasts with the geometry of a convex polygon.
Interior angle: An angle inside a polygon formed by two adjacent sides. The angles at a polygon’s vertices help determine its shape and area.
Interior angle sum: The sum of a polygon’s interior angles, equal to (n−2) × 180° for a simple n-sided polygon. Triangulating a polygon explains why its angle sum depends on its number of sides.

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