headroom

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun Space above one's head, as in a motor vehicle, above a doorway, or in a tunnel; clearance.

from The Century Dictionary.

• noun The vertical distance from a floor to the ceiling or beams above it: so named because this distance determines whether or not a person has room for his head when standing or walking on the floor.
• noun Specifically, the space left above a stair by means of which the head of the person ascending the stair is kept free from striking or coming too near the superstructure.

from the GNU version of the Collaborative International Dictionary of English.

• noun (Arch.), Mostly Brit. See headway, 2.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun the vertical clearance above someone's head, as in a tunnel, doorway etc
• noun electronics the ability of a system to reproduce loud sounds free of distortion; dynamic headroom
• noun the distance between the actual performance of an algorithm to its maximum possible performance

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun vertical space available to allow easy passage under something
• noun the capacity of a system to reproduce loud sounds without distortion