Source : Webster's Revised Unabridged Dictionary (1913)

Fieldpiece \Field"piece`\, n.
   A cannon mounted on wheels, for the use of a marching army; a
   piece of field artillery; -- called also {field gun}.

Source : WordNet®

field gun
     n : movable artillery (other than antiaircraft) used by armies
         in the field (especially for direct support of front-line
         troops) [syn: {field artillery}]