Any engine propelled by expelling a high speed fluid jet (jet propulsion), such as a rocket, turbojet, turbofan, ramjet, etc.
An engine that obtains thrust by taking in air at the front, using it to burn fuel, then ejecting the hot combustion products at the rear through a propulsive nozzle.
A jet engine in which forward motion forces air into an inlet, compressing it (as opposed to having a pump type device compressing the air for combustion with fuel), and where combustion is subsonic.