headroom

Short definition from Wiktionary

Noun

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