die·sel I. \ˈdēzəl, -ēsəl\adjective Usage: sometimes capitalized Etymology: after Rudolf Diesel died 1913 German mechanical engineer and inventor 1.: relating to the diesel cycle or engine 2.: equipped with or driven by a diesel engine < a diesel truck > II. noun Usage: sometimes capitalized 1.: diesel engine 2.: a vehicle (as a truck, locomotive, or ship) driven by a diesel engine