headroom

The vertical measurement, top to bottom, for example for clearance under a bridge.

The distance between the actual performance of an algorithm and its maximum possible performance.

Synonym of headspace (“space between the top of the contents of a container and its seal”).

The vertical clearance above someone's head, as in a tunnel, doorway etc; room (space) for one's head.

The ability of a system to reproduce loud sounds free of distortion; dynamic headroom.