Haki

Haki is the strength of a person's spirit made usable. Everyone carries it and almost nobody trains it, which is rather the point. Fruit powers get the poster art, but the thing separating the top of this world from the middle is invisible, comes from character instead of luck, and can be worked at by anybody willing to be beaten up for long enough.

It splits three ways. Armament Haki coats a body or a blade in something like black armour, hardening it and letting it strike targets a normal punch would pass straight through, which is how anyone hurts a man made of smoke or rubber. Observation Haki reads presence and intent, so a trained user feels an attack before it lands and, at the far end of the skill, sees a few seconds ahead.

The third kind cannot be taught. Conqueror's Haki turns up in roughly one person in a million and floors anyone with a weaker will simply by being aimed at them. It can be refined but never learned, and it tends to appear in people the story is about to promote. Oda held the whole concept back for hundreds of chapters, then used it to explain fights readers had already watched.