A prism is a solid whose ends, or bases, are parallel congruent polygons, and whose sides or faces are parallelograms. A straight line moving parallel to itself, with its extremities traveling around the outlines of plane figures, generates the prism.
Prisms are named according to the shape of the ends or bases. A prism with a square base is called a square prism and a prism with hexagonal base is called a hexagonal prism. Similarly, when the ends or bases of a prism are parallelograms, the prism is called a parallelepiped.
As shown in the figure, the sides or faces of the prism are , , etc. These faces are parallelograms. If they were rectangles, angle would be and the prism would be a right prism. A prism that is not a right prism is called an oblique prism. One side of one of these parallelograms of these prisms is called a lateral edge.
The altitude of a prism is the distance between the planes of the two bases (i.e. ). In a right prism, the altitude is the same length as a lateral edge, and this is not true for an oblique prism.