The Collaborative International Dictionary of English v.0.48:

Headway \Head"way`\ (-w[^u]rk`), n.
   1. The progress made by a ship in motion; hence, progress or
      success of any kind.
      [1913 Webster]

   2. (Arch.) Clear space under an arch, girder, and the like,
      sufficient to allow of easy passing underneath; clearance;
      headroom.
      [1913 Webster]

WordNet (r) 3.0 (2006):

headway
    n 1: vertical space available to allow easy passage under
         something [syn: headroom, headway, clearance]
    2: forward movement; "the ship made little headway against the
       gale" [syn: headway, head]