• WordNet 3.6
    • n headroom the capacity of a system to reproduce loud sounds without distortion
    • n headroom vertical space available to allow easy passage under something
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Webster's Revised Unabridged Dictionary
    • n Headroom (Arch) See Headway, 2.
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Century Dictionary and Cyclopedia
    • n headroom 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.
    • n headroom 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. The term may apply either to the vertical distance from the nosing of the step below to the ceiling or any cross-beam or the like above, or to the distance measured out diagonally from the nosing of a step.
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In literature:

All Mother insists on is plenty of headroom.
"Love Conquers All" by Robert C. Benchley
A light airy greenhouse is their proper place, and they must have ample headroom.
"The Culture of Vegetables and Flowers From Seeds and Roots, 16th Edition" by Sutton and Sons
The headroom between the subway roof and the surface of the street was 4.75 feet.
"The New York Subway" by Anonymous
Since there was not enough headroom upwards it had dug downwards.
"Letters from France" by C. E. W. Bean
This roof was level with the bank of the hedge and gave about four feet of headroom.
"The Fifth Battalion Highland Light Infantry in the War 1914-1918" by F.L. Morrison
The result was a pleasant airy compartment with headroom even for Roger.
"The Forbidden Trail" by Honoré Willsie
It has more headroom, and therefore easier working facilities.
"Mushrooms: how to grow them" by William Falconer
Rawson could have stood upright in it with easily another two feet of headroom to spare.
"Two Thousand Miles Below" by Charles Willard Diffin
Such a feature gave additional ceiling space and more headroom.
"Virginia Architecture in the Seventeenth Century" by Henry Chandlee Forman

In news:

The BE-550 lightweight bulkhead evaporator for small to mid-size shuttle and school bus applications to help ensure passenger comfort in vehicles where headroom clearance is an issue.
Compounder 's Low Headroom Doesn't Stunt Its Growth.
I like to graduate the distance between cleats, so the bottom boxes have more headroom.
"Jon," he asked, "could you explain Max Headroom to me".
Jay Rosen on SXSW, Future of News Context, Prophecy of 'Max Headroom '.
Compounder's Low Headroom Doesn't Stunt Its Growth.
Doosan says cab vibration has been reduced 20 percent and the cabin size has increased 6.1 percent for more headroom.
0Driven by customer specifications, the SpaceSaver is a patented, low-profile, compact design for high-efficiency dust collection where there is limited footprint space or available headroom.
Rear headroom is an issue, too, thanks to the convertible 's lower, sloped roofline.
Inside, the Forester 's increase in size translates into more headroom and legroom front and rear, to the point where four six-footers should find accommodations plenty comfortable.
And every time I go for a lunchtime walk, I remember the crazy Brit writers who were inspired to create an eponymous Oriental character for Max Headroom.
The Russell Compact screener is ideal for high-capacity check-screening, sifting or grading, especially where headroom is restricted.
It features a low headroom design and efficient one person operation.
Legroom and Headroom For Taller Drivers.
Narrow your used car search by researching the car specs including engine performance options, interior upgrades, headroom, cargo capacity and more.

In science:

In typical atmospheric conditions at astronomical observatories, this compensation length is greater than 5 sigma of the amplitude of the turbulence induced optical error and therefore results in significant correction headroom.
Bringing the Visible Universe into Focus with Robo-AO
Hence, the headroom in the cryogenic plant can be traded off to achieve higher gradients.
Summary of Discussion Question 4: Energy Expandability of a Linear Collider
As of the time of writing, travelling-wave cavities have been successfully operated with unloaded gradients as high as 80 MV/m; it is desirable to reach 90 MV/m in order to allow more ‘headroom’.
Summary of Discussion Question 4: Energy Expandability of a Linear Collider
This is about an order of magnitude below the threshold at which a non-negligible charge transfer inefficiency (of order 10−4) results from charge-trapping by damage centres. A number of promising ideas offer the possibility of further headroom.
R&D for a CCD Vertex Detector for the High Energy Linear e+e- Collider