headway

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

• noun Forward movement or the rate of forward movement, especially of a ship.
• noun Progress toward a goal.
• noun The clear vertical space beneath a ceiling or archway; clearance.
• noun The distance in time or space that separates two vehicles traveling the same route.

from The Century Dictionary.

• noun Motion ahead or forward; force or amount of such motion; rate of progress: said specifically of a ship, but applied to all kinds of progress, literally or figuratively.
• noun In architecture, clear space in height, as from a floor to a ceiling, or from the ground to the crown of an arch; specifically, the perpendicular distance from any step or landing of a stair to the ceiling.
• noun In coal-mining, a cross-heading.
• noun In railroading, the. time which elapses after one train passes a certain point before the following train passes that point.

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

• noun The progress made by a ship in motion; hence, progress or success of any kind.
• noun (Arch.) Clear space under an arch, girder, and the like, sufficient to allow of easy passing underneath; clearance; headroom.

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

• noun Movement ahead or forward.
• noun nautical Forward motion, or its rate.
• noun countable, transport The interval of time or distance between the fronts of two vehicles (e.g. buses) moving in succession in the same direction, especially along the same pre-determined route.
• noun uncountable, figuratively Progress toward a goal.
• noun countable The clearance beneath an object, such as an arch, ceiling or bridge; headroom.
• noun A cross-heading.

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

• noun forward movement
• noun vertical space available to allow easy passage under something