The Collaborative International Dictionary of English v.0.48:

Commutation \Com`mu*ta"tion\, n. [L. commutatio: cf. F.
   commutation.]
   1. A passing from one state to another; change; alteration;
      mutation. [R.]
      [1913 Webster]

            So great is the commutation that the soul then hated
            only that which now only it loves.    --South.
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   2. The act of giving one thing for another; barter; exchange.
      [Obs.]
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            The use of money is . . . that of saving the
            commutation of more bulky commodities. --Arbuthnot.
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   3. (Law) The change of a penalty or punishment by the
      pardoning power of the State; as, the commutation of a
      sentence of death to banishment or imprisonment.
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            Suits are allowable in the spiritual courts for
            money agreed to be given as a commutation for
            penance.                              --Blackstone.
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   4. A substitution, as of a less thing for a greater, esp. a
      substitution of one form of payment for another, or one
      payment for many, or a specific sum of money for
      conditional payments or allowances; as, commutation of
      tithes; commutation of fares; commutation of copyright;
      commutation of rations.
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   5. regular travel from a place of residence to a place where
      one's daily work is performed; commuting. Most often, such
      travel is performed between a suburb and a nearby city.
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   Angle of commutation (Astron.), the difference of the
      geocentric longitudes of the sun and a planet.

   Commutation of tithes, the substitution of a regular
      payment, chargeable to the land, for the annual tithes in
      kind.

   Commutation ticket, a ticket, as for transportation, which
      is the evidence of a contract for service at a reduced
      rate. See 2d Commute, 2.
      [1913 Webster]

WordNet (r) 3.0 (2006):

commutation
    n 1: the travel of a commuter [syn: commutation, commuting]
    2: a warrant substituting a lesser punishment for a greater one
    3: (law) the reduction in severity of a punishment imposed by
       law [syn: commutation, re-sentencing]
    4: the act of putting one thing or person in the place of
       another: "he sent Smith in for Jones but the substitution
       came too late to help" [syn: substitution, exchange,
       commutation]

Moby Thesaurus II by Grady Ward, 1.0:

82 Moby Thesaurus words for "commutation":
   agency, alternation, amends, atonement, balancing,
   battledore and shuttlecock, change, compensation, cooperation,
   counteraction, counterbalancing, counterchange, course, cross fire,
   crossing, delegation, deputation, deputyship, displacement,
   exchange, expiation, give-and-take, globe-trotting, going,
   indemnification, indemnity, interchange, intermutation, interplay,
   journeying, lex talionis, locomotion, measure for measure, motion,
   movement, moving, mutual admiration, mutual support,
   mutual transfer, mutuality, offsetting, passage, permutation,
   power of attorney, progress, quid pro quo, reciprocality,
   reciprocation, reciprocity, recompense, rectification, redress,
   reparation, repayment, replacement, representation, restitution,
   retaliation, revenge, satisfaction, something for something,
   subrogation, substitution, supersedence, superseding, supersedure,
   supersession, supplantation, supplanting, supplantment, switch,
   tit for tat, tourism, touristry, traject, trajet, transit,
   transposal, transposition, travel, traveling, vicariousness



Bouvier's Law Dictionary, Revised 6th Ed (1856):

COMMUTATION, punishments. The change of a punishment to which a person has
been condemned into a less severe one. This can be granted only by the
executive authority in which the pardoning power resides.