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