The Collaborative International Dictionary of English v.0.48:

Howitzer \How"itz*er\, n. [G. haubitze, formerly hauffnitz,
   Bohem. haufnice, orig., a sling.] (Mil.)
   (a) A gun so short that the projectile, which was hollow,
       could be put in its place by hand; a kind of mortar.
       [Obs.]
   (b) A short, light, largebore cannon, usually having a
       chamber of smaller diameter than the rest of the bore,
       and intended to throw large projectiles with
       comparatively small charges.
       [1913 Webster]

WordNet (r) 3.0 (2006):

howitzer
    n 1: a muzzle-loading high-angle gun with a short barrel that
         fires shells at high elevations for a short range [syn:
         mortar, howitzer, trench mortar]