The Collaborative International Dictionary of English v.0.48:

Trunnion \Trun"nion\, n. [OF. trognon the stock, stump, or truck
   of a tree, F. trognon a core, stalk, fr. tron a trunk, stem.
   Cf. Trunk.]
   1. (Gun.) A cylindrical projection on each side of a piece,
      whether gun, mortar, or howitzer, serving to support it on
      the cheeks of the carriage. See Illust. of Cannon.
      [1913 Webster]

   2. (Steam Engine) A gudgeon on each side of an oscillating
      steam cylinder, to support it. It is usually tubular, to
      convey steam.
      [1913 Webster]

   Trunnion plate (Gun.), a plate in the carriage of a gun,
      mortar, or howitzer, which covers the upper part of the
      cheek, and forms a bearing under the trunnion.

   Trunnion ring (Gun.), a ring on a cannon next before the
      trunnions. [R.]
      [1913 Webster]