Def. 13. A prism is a solid figure contained by planes two of which, namely those which are opposite, are equal, similar, and parallel, while the rest are parallelograms.
In the diagram below, ABCD is a pyramid with vertex D and triangular base ABC. Since it's a tetrahedron, it's still a triangular pyramid when any of the other three sides is considered the base. Also, EFGHKL is a prism with opposite triangular sides EFG and HKL.Book XI Introduction - Definition 11 - Definitions 14 through 17.