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.
Examples
Non-examples
Things that may be mistaken for examples.
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The unit circle, for the closed unit disk in $\reals^2$
High school
The unit circle is not an edge of a polygon, because the closed unit disk has no straight sides.