Definition
Face of a solid
A face is a flat two-dimensional surface region that forms part of a three-dimensional solid.
What it means
Think of a box. Each flat panel you could cover with one sheet of paper is a face. Faces meet along edges. A cylinder has two flat circular faces, while its curved side is a curved surface rather than a face in this sense.
Remember it
A face is one flat patch on a solid, like a panel of a box.
Example
A triangular prism has five faces: two triangular faces at the ends and three rectangular faces around the sides. Following each flat region once gives the total without counting an edge as a face.
Watch for
A sphere has no flat faces. A cylinder’s two flat circles can be called faces, but its curved side is not a face under this convention.
Common mix-up
A face is a flat surface region; an edge is the line where faces meet, and a vertex is a corner point.
Exact meaning
A face of a solid is a flat two-dimensional region on its surface. On a polyhedron, each face is a polygon and faces meet along edges; curved parts are surfaces, not faces in this card’s strict sense.