It depends on the context: for path-tracing (uni-directional / bi-directional without photon mapping / VCM), "forward" generally means starting at the camera (e

It depends on the context: for path-tracing (uni-directional / bi-directional without photon mapping / VCM), "forward" generally means starting at the camera (eye) and bouncing around the scene. "backward" generally means starting at the lights in this context (for bi-directional). Forward in the photon context generally comes from the fact that multiple bounces didn't used to be done, you generally sent out photons from the lights, they were stored, and then a final gather was done to create the image.