The Collaborative International Dictionary of English v.0.48:

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.
   [1913 Webster]

WordNet (r) 3.0 (2006):

field gun
    n 1: movable artillery (other than antiaircraft) used by armies
         in the field (especially for direct support of front-line
         troops) [syn: field artillery, field gun]