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.]
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            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.
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The Collaborative International Dictionary of English v.0.48:

Commutation ticket \Com`mu*ta"tion tick"et\
   A ticket for transportation at a reduced rate in
   consideration of some special circumstance, as increase of
   travel; specif., a ticket for a certain number of, or for
   daily, trips between neighboring places at a reduced rate,
   such as are commonly used by those doing business in a city
   and living in a suburb. Commutation tickets are excepted from
   the prohibition against special rates contained in the
   Interstate Commerce Act of Feb. 4, 1887 (--24 Stat. 379), and
   in 145 U. S. 263 it was held that party tickets were also
   excepted as being "obviously within the commuting principle."
   [Webster 1913 Suppl.]

WordNet (r) 3.0 (2006):

commutation ticket
    n 1: a ticket good for several trips or to attend a season of
         entertainments; sold at a reduced rate [syn: commutation
         ticket, season ticket]