flat surfaces on a rectangular prism are called faces.
it is so because those surfaces are of polyhedron shape means that their spherical surfaces are in front and they have a general outward appearance. Face is a Latin word( facies) meaning in front.
polyhedron shape is like:
In geometric objects faces are surfaces of some objects which can be viewed and meet at edges. Rectangular prism has surfaces which can be seen. Between surfaces always there are edges. Secondly face have to be flat surface. Every solid figure has faces. They vary only in shape. Rectangular prism has four flat rectangular surfaces and two square flat surfaces. Each of them fulfill conditions for being a face.