The Collaborative International Dictionary of English v.0.48:

Windage \Wind"age\, n. [From Wind air in motion.]
   [1913 Webster]
   1. (Gun.) The difference between the diameter of the bore of
      a gun and that of the shot fired from it.
      [1913 Webster]

   2. The sudden compression of the air caused by a projectile
      in passing close to another body.
      [1913 Webster]

WordNet (r) 3.0 (2006):

windage
    n 1: the retarding force of air friction on a moving object
    2: the space between the projectile of a smoothbore gun and the
       surface of the bore of the gun
    3: exposure to the wind (as the exposed part of a vessel's hull
       which is responsible for wind resistance) [syn: windage,
       wind exposure]
    4: the deflection of a projectile resulting from the effects of
       wind [syn: windage, wind deflection]