tetrahedron
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A closed three-dimensional figure formed by four or more polygons that intersect only at their edges is called a polyhedron. It contains flat faces, straight edges, and sharp corners or vertices.
A rectangle is a two-dimensional shape, so it doesn't have faces in the way that three dimensional shapes have. A rectangle has four edges and four corners.
If it is a 2-dimensional figure then it is a quadrilateral. If it is a 3-dimensional figure then it is a tetrahedron.
A tetrahedron is a three-dimensional figure with four equilateral triangles. Every pyramid with a triangle as the base is a tetrahedron.
A three sided pyramid has four sides and six edges (think four sided die used in role playing games).