A closed figure composed of three or more line segments on the same plane but not on the same straight line, which is connected end to end and does not intersect, is called a polygon. A figure composed of many line segments on different planes that are connected end to end and do not intersect is also called a polygon, which is a generalized polygon.
There are at least three line segments that make up a polygon, and a triangle is the simplest polygon. Each line segment that constitutes a polygon is called an edge of the polygon; The common endpoint of two adjacent line segments is called the vertex of the polygon; The angle formed by two adjacent sides of a polygon is called the inner angle of the polygon; The line segment connecting two nonadjacent vertices of a polygon is called the diagonal of the polygon.
The angle formed by the extension line with one side of the inner angle of a polygon opposite to the other side is called the outer angle of the polygon.
Take an outer corner of the polygon at each vertex, and their sum is called the sum of the outer corners of the polygon.
Polygons can also be divided into regular polygons and non-regular polygons. Regular polygons have equal sides and equal internal angles.