in flight a powered aircraft can be considered as being acted on by four forces: lift, weight, thrust, and drag. "thrust" is the force generated by the engine (whether that engine be a jet engine, a propeller, or -- in exotic cases such as the x-15 -- a rocket) and acts in a forward direction for the purpose of overcoming drag. "lift" acts perpendicular to the vector representing the aircraft's velocity relative to the atmosphere. "drag" acts parallel to the aircraft's velocity vector, but in the opposite direction because drag resists motion through the air. "weight" acts through the aircraft's centre of gravity, towards the centre of the earth.