By looking at the name, I would think that they're the opposite counterpart to each other. Especially, they share the same VisibilitySpec to select an element and specify the conditions to meet.

However, the "visible" trigger fires when the selected element enters viewport; but the "hidden" trigger fires when user leaves the page (vs selected element leaves viewport).

It will be more self-explained if we rename "hidden" to "pagehidden" (or something similar).