Headroom

Noun

1. The vertical clearance above someone's head, as in a tunnel, doorway etc; room (space) for one's head.
2. The vertical measurement, top to bottom, for example for clearance under a bridge.
3. The ability of a system to reproduce loud sounds free of distortion; dynamic headroom.
4. The distance between the actual performance of an algorithm and its maximum possible performance.
5. Synonym of headspace (“space between the top of the contents of a container and its seal”).