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A prism is a special kind of polyhedron. It has two ends which are exactly the same shape and size. These end shapes are parallel to each other, are separated by rectangles and are often squares.
If a prism is sliced parallel to one of its ends then the new exposed ends will be exactly the same as the old ones. A prism is the same shape and size all the way through.

A solid with two parallel congruent polygonal ends (with their corresponding sides parallel) and a number of parallelogram faces joining the two ends. If the ends and the other faces are perpendicular, then it is a right prism. If the ends of a right prism are regular polygons, then it is a regular prism.
A triangular prism has triangular ends; a square prism has square ends, etc.