Source : Webster's Revised Unabridged Dictionary (1913)

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

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

Source : WordNet®

windage
     n 1: the retarding force of air friction on a moving object
     2: bore-to-projectile difference in diameter
     3: exposure to the wind (as the exposed part of a vessel's hull
        which is responsible for wind resistance) [syn: {wind
        exposure}]
     4: the deflection of a projectile resulting from the effects of
        wind [syn: {wind deflection}]