An engine was a type of device that would convert one form of energy into another, often as a propulsion system.
One of the biggest Human discoveries of the 21st century was the warp drive engine by Zefram Cochrane, which he called a "space warp generator". (Star Trek: First Contact) Within a century, Human scientists had managed to increase the speed attained until they created the warp five engine. (ENT: "First Flight", "Broken Bow")
By the 23rd century, several types of engines were used to help propel starships including the Avidyne engines, used as impulse drives. (TNG: "Peak Performance") In order to drive the warp engine, antimatter engines and matter engines helped control the matter-antimatter reaction assembly. (TAS: "One of Our Planets Is Missing")