The Collaborative International Dictionary of English v.0.48:

Eccentric \Ec*cen"tric\ ([e^]k*s[e^]n"tr[i^]k), n.
   1. A circle not having the same center as another contained
      in some measure within the first.
      [1913 Webster]

   2. One who, or that which, deviates from regularity; an
      anomalous or irregular person or thing.
      [1913 Webster]

   3. (Astron.)
      (a) In the Ptolemaic system, the supposed circular orbit
          of a planet about the earth, but with the earth not in
          its center.
      (b) A circle described about the center of an elliptical
          orbit, with half the major axis for radius. --Hutton.
          [1913 Webster]

   4. (Mach.) 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.
      [1913 Webster]

   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.
      [1913 Webster]