= Solution
For solid modelling, a <polyhedron> is a solid whose boundary consists of finitely many planar <polygons>, called faces, meeting along straight edges and at <vertices of a polyhedron>. A face is not merely a list of arbitrary spatial <points>: its boundary must lie in one <plane> and bound a valid planar region. The boundary is closed and consistently oriented so that inside and outside are defined.
For example, a <tetrahedron> has four triangular faces, six edges and four <vertices of a polyhedron>. A more general <polyhedron> need not be <convex> and can have handles or cavities. The concise definition is
$$
\boxed{\text{a solid enclosed by finitely many planar polygonal faces}.}
$$
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