The Collaborative International Dictionary of English v.0.48:

Mechanism \Mech"an*ism\, n. [Cf. F. m['e]canisme, L. mechanisma.
   See Mechanic.]
   1. The arrangement or relation of the parts of a machine; the
      parts of a machine, taken collectively; the arrangement or
      relation of the parts of anything as adapted to produce an
      effect; as, the mechanism of a watch; the mechanism of a
      sewing machine; the mechanism of a seed pod.
      [1913 Webster]

   2. The series of causal relations that operate to produce an
      effect in any system; as, the mechanism of a chemical
      reaction.
      [PJC]

   3. Mechanical operation or action.
      [1913 Webster]

            He acknowledges nothing besides matter and motion;
            so that all must be performed either by mechanism or
            accident.                             --Bentley.
      [1913 Webster]

   4. (Kinematics) An ideal machine; a combination of movable
      bodies constituting a machine, but considered only with
      regard to relative movements.
      [1913 Webster]

WordNet (r) 3.0 (2006):

mechanism
    n 1: the atomic process that occurs during a chemical reaction;
         "he determined unique mechanisms for the photochemical
         reactions" [syn: mechanism, chemical mechanism]
    2: the technical aspects of doing something; "a mechanism of
       social control"; "mechanisms of communication"; "the
       mechanics of prose style" [syn: mechanism, mechanics]
    3: a natural object resembling a machine in structure or
       function; "the mechanism of the ear"; "the mechanism of
       infection"
    4: (philosophy) the philosophical theory that all phenomena can
       be explained in terms of physical or biological causes
    5: device consisting of a piece of machinery; has moving parts
       that perform some function