The mechanical appliance by means of which a vessel is
   guided or steered when in motion. It is a broad and flat blade made of
   wood or iron, with a long shank, and is fastened in an upright
   position, usually by one edge, to the sternpost of the vessel in such a
   way that it can be turned from side to side in the water by means of a
   tiller, wheel, or other attachment.