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

Moby Thesaurus II by Grady Ward, 1.0:

149 Moby Thesaurus words for "mechanism":
   Charlie McCarthy, Marxism, action, agency, agent, ancilla,
   animalism, apparatus, appliance, approach, arrangement, art,
   atomism, automatic switch, behaviorism, channel selector,
   check valve, clockworks, commonsense realism, contour follower,
   contrivance, control valve, convenience, craft, creature, device,
   dialectical materialism, drive, drive train, dummy, dupe,
   earthliness, electronic control, emergency control, empiricism,
   engine, enginery, epiphenomenalism, expedient, facility,
   finder switch, fixture, flowmanostat, gear, go-between,
   going between, handmaid, handmaiden, historical materialism,
   hylomorphism, hylotheism, hylozoism, implement, innards,
   instrument, instrumentality, interagent, intermediary,
   intermediate, intermediation, intermedium, internal selector,
   lever, machine, machinery, materialism, means, mechanical aid,
   mechanical device, mechanics, mediation, mediator, medium, method,
   midwife, minion, monism, motion, motive power, motor, movement,
   movements, natural realism, naturalism, new realism, organ,
   organization, pawn, physicalism, physicism, plaything,
   positive philosophy, positivism, power plant, power source,
   power train, pragmaticism, pragmatism, procedure, process, puppet,
   realism, recourse, rectifier, register regulator,
   representative realism, resort, safety control, safety fuse,
   safety stop, safety switch, safety valve, science, secularism,
   selector switch, sequence switch, servant, service, servo valve,
   servomechanism, skill, slave, stooge, structure, substantialism,
   system, technic, technical know-how, technical knowledge,
   technical skill, technics, technique, technology, temporality,
   thermoswitch, timer, tool, toy, utility, vehicle,
   voltage regulator, voltage stabilizer, watchworks, way, wheels,
   wheels within wheels, workings, works, worldliness