Source : Webster's Revised Unabridged Dictionary (1913)

Clearance \Clear"ance\ (-ans), n.
   1. The act of clearing; as, to make a thorough clearance.

   2. A certificate that a ship or vessel has been cleared at
      the customhouse; permission to sail.

            Every ship was subject to seizure for want of
            stamped clearances.                   --Durke

   3. Clear or net profit. --Trollope.

   4. (Mach.) The distance by which one object clears another,
      as the distance between the piston and cylinder head at
      the end of a stroke in a steam engine, or the least
      distance between the point of a cogwheel tooth and the
      bottom of a space between teeth of a wheel with which it
      engages.

   {Clearance space} (Steam engine), the space inclosed in one
      end of the cylinder, between the valve or valves and the
      piston, at the beginning of a stroke; waste room. It
      includes the space caused by the piston's clearance and
      the space in ports, passageways, etc. Its volume is often
      expressed as a certain proportion of the volume swept by
      the piston in a single stroke.

Source : WordNet®

clearance
     n 1: the distance by which one thing clears another; the space
          between them
     2: vertical space available to allow easy passage under
        something [syn: {headroom}, {headway}]
     3: permission to proceed; "the plane was given clearance to
        land"