No. A prism is only one kind of polyhedron: it has two identical, parallel polygonal bases joined by rectangular or parallelogram faces. Polyhedra also include pyramids (one base with triangular faces meeting at an apex), Platonic solids such as the tetrahedron and octahedron, and many other shapes that do not meet the two-parallel-bases rule.
So every prism is a polyhedron, but not every polyhedron is a prism. A square pyramid and a tetrahedron are everyday counterexamples.