Edge of a polygon or polyhedron
Elementary
A polygon is a 2D shape with straight sides that form a loop. Each side is called an edge of the polygon.
High school
In 2D, an edge of the polygon is a line segment between two adjacent vertices (one of its sides). In 3D, an edge of the polyhedron is a segment where two of its faces meet, such as the 12 line segments separating the faces of a cube.