headroom

noun

1. (countable, uncountable)The vertical clearance above someone's head, as in a tunnel, doorway etc; room (space) for one's head.
2. (countable, uncountable)The vertical measurement, top to bottom, for example for clearance under a bridge.

   “With a rare and beautiful ease one can now ring up a boffin, as I did the other day, and say: "As a price for not opposing our Parliamentary Powers for a new marshalling yard, the Council at X demands that the bridge over X Lane shall have 16 ft. 6 in. headroom. This means steepening our gradient from 1 in 70 to 1 in 65 for half a mile on a 20-chain curve. What difference will this make to the loads of Type "2", "3" and "4" diesels please?". Back comes the answer.”

   “The new bridge (6) gives headroom of 13ft at high tide, sufficient for present-day river traffic.”

3. (countable, uncountable)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).”

4. (countable, uncountable)The distance between the actual performance of an algorithm and its maximum possible performance.
5. (countable, uncountable)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 […]”