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.

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.]

Source: Webster's Revised Unabridged Dictionary (1913)