engine

(noun) motor that converts thermal energy to mechanical work.

(noun) something used to achieve a purpose. Example: "An engine of change."

(noun) a wheeled vehicle consisting of a self-propelled engine that is used to draw trains along railway tracks. Synonyms: locomotive, locomotive engine, railway locomotive.

(noun) an instrument or machine that is used in warfare, such as a battering ram, catapult, artillery piece, etc.. Example: "Medieval engines of war."