Merriam-Webster Collegiate Dictionary - windage

 noun  Etymology: 1wind  Date: circa 1710  1.  a. the space between the projectile of a smoothbore gun and the surface of the bore  b. the difference between the diameter of the bore of a muzzle-loading rifled cannon and that of the projectile cylinder  2.  a. the amount of sight deflection necessary to compensate for wind displacement in aiming a gun  b.  (1) the influence of the wind in deflecting the course of a projectile  (2) the amount of deflection due to the wind  3. the surface exposed (as by a ship) to the wind