headroom

/HH EH1 D R UW0 M/
noun
  1. 1

    The vertical clearance above someone's head, as in a tunnel, doorway etc.

  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.

Source: https://en.wiktionary.org/wiki/headroom