The four forces of flight are; thrust-the movement of the airplane moving foreword, drag-the force of the plane being pulled backwards, gravity-the force of pulling the plane down, and last, lift-the force pulling the plane up. All of these are equally important in the processes of flight.
My definition and rules of flight are; it must be supported be air pressure, it cannot be sheer force moving it, and it must move through the air without something pushing or pulling it (with the exception of a propeller, engine ect).