What is a face on a triangular prism?

It is no different than on any other 3D figures. A face is a surface forming part of the outside of an object, such as "he examined all faces of the crystal." On a triangular prism, there are 5 faces. Two triangular faces on opposite ends, and thin 3 rectangular faces to connect them. To see what a triangular prism looks like, just google it on googleimages. ! :) :) :)