headroom

noun5

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.

   • The new bridge (6) gives headroom of 13ft at high tide, sufficient for present-day river traffic. — 1963
3. •
   Synonym of headspace (“space between the top of the contents of a container and its seal”).

   • For whole pickles and fruit preserves, leave no less than ½ inch of headroom at the top of the jars […] — 2014
4. •
   The ability of a system to reproduce loud sounds free of distortion; dynamic headroom.

   • […] the difference between signal-to-noise ratio and overload reserve (headroom) as well as noise (safety) margin (footroom). — 2000
5. •
   The distance between the actual performance of an algorithm and its maximum possible performance.