Cog, n. [Cf. Sw. kugge a cog, or W. cocos the cogs of a wheel.] 1. (Mech.) A tooth, cam, or catch for imparting or receiving motion, as on a gear wheel, or a lifter or wiper on a shaft; originally, a separate piece of wood set in a mortise in the face of a wheel.
2. (Carp.) (a) A kind of tenon on the end of a joist, received into a notch in a bearing timber, and resting flush with its upper surface. (b) A tenon in a scarf joint; a coak. Knight.
3. (Mining.) One of the rough pillars of stone or coal left to support the roof of a mine.