Definition
Edge
An edge is a straight line segment where two faces of a polyhedron meet.
Continue from this idea into connected AXIO math.
What it means
On a cube, trace any straight seam where one square face meets another. That seam is an edge. Faces meet along edges, and edges meet at vertices. Count each physical segment once, even though it belongs to two faces.
Remember it
Faces meet along edges; edges meet at vertices.
Example
A cube has 4 edges around the top face, 4 around the bottom face, and 4 joining top to bottom. That makes 12 edges altogether.
Watch for
A side of a flat polygon is not called an edge in this AXIO sense, and a curved rim is not a straight polyhedron edge.
Common mix-up
A face is a flat region; an edge is the segment where two faces meet; a vertex is a point where edges meet.
Exact meaning
In a polyhedron, an edge is a straight line segment where two polygonal faces meet. Its endpoints are vertices. This card uses the polyhedron sense, so the boundary of a 2-D polygon is called a side rather than an edge.