Source : Webster's Revised Unabridged Dictionary (1913)

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

            So great is the commutation that the soul then hated
            only that which now only it loves.    --South.

   2. The act of giving one thing for another; barter; exchange.
      [Obs.]

            The use of money is . . . that of saving the
            commutation of more bulky commodities. --Arbuthnot.

   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.

            Suits are allowable in the spiritual courts for
            money agreed to be given as a commutation for
            penance.                              --Blackstone.

   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.

   {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.

Source : WordNet®

commutation
     n 1: the travel of a commuter [syn: {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: {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}]