Trunnion

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.