Possible interaction in a game which is robust, and presented clearly to player in a way that would allow them to perform said action given the current context and their expertise. Natural actions are those which players would describe as fair, from their perspective.
“Everything you do in FEZ is a natural action. Your character moves in predictable, reproducible ways which are shown to be useful to your progress. Grabbing ledges is effortless, in large part, because of how well the visuals telegraph what the game considers a ledge in the first place.”
See also: ARCANE ACTION