Eccentric

n.
  1. A circle not having the same center as another contained in some measure within the first.
  2. One who, or that which, deviates from regularity; an anomalous or irregular person or thing.
  3. (Astron.) A disk or wheel so arranged upon a shaft that the center of the wheel and that of the shaft do not coincide. It is used for operating valves in steam engines, and for other purposes. The motion derived is precisely that of a crank having the same throw.
    1. In the Ptolemaic system, the supposed circular orbit of a planet about the earth, but with the earth not in its center.
    2. A circle described about the center of an elliptical orbit, with half the major axis for radius.
    Back eccentric ,
    the eccentric that reverses or backs the valve gear and the engine.
    Fore eccentric ,
    the eccentric that imparts a forward motion to the valve gear and the engine.