headroom

head +‎ room

headroom (countable and uncountable, plural headrooms)

1. The vertical clearance above someone's head, as in a tunnel, doorway etc.
2. The vertical measurement, top to bottom, for example for clearance under a bridge.
   • 1960 April, G. F. Fiennes, “Unpunctuality - the cause and the cure”, in Trains Illustrated, page 244:

     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.

   • 1963 April, “Chepstow Bridge is rebuilt”, in Modern Railways, page 265, photo caption:

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

3. (electronics) The ability of a system to reproduce loud sounds free of distortion; dynamic headroom.
   • 2000, Wolfgang Ahnert; Frank Steffen, Sound Reinforcement Engineering: Fundamentals and Practice, page 177:

     […] the difference between signal-to-noise ratio and overload reserve (headroom) as well as noise (safety) margin (footroom).

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

• footroom
• legroom

• Moorhead