The Collaborative International Dictionary of English v.0.48:

Substitution \Sub`sti*tu"tion\, n. [L. substitutio: cf. F.
   substitution.]
   1. The act of substituting or putting one person or thing in
      the place of another; as, the substitution of an agent,
      attorney, or representative to act for one in his absense;
      the substitution of bank notes for gold and silver as a
      circulating medium.
      [1913 Webster]

   2. The state of being substituted for another.
      [1913 Webster]

   3. The office or authority of one acting for another;
      delegated authority. [R.] --Shak.
      [1913 Webster]

   4. (Civil Law) The designation of a person in a will to take
      a devise or legacy, either on failure of a former devisee
      or legatee by incapacity or unwillingness to accept, or
      after him. --Burrill.
      [1913 Webster]

   5. (Theol.) The doctrine that Christ suffered vicariously,
      being substituted for the sinner, and that his sufferings
      were expiatory.
      [1913 Webster]

   6. (Chem.) The act or process of substituting an atom or
      radical for another atom or radical; metathesis; also, the
      state of being so substituted. See Metathesis.
      [1913 Webster]

WordNet (r) 3.0 (2006):

substitution
    n 1: an event in which one thing is substituted for another;
         "the replacement of lost blood by a transfusion of donor
         blood" [syn: substitution, permutation,
         transposition, replacement, switch]
    2: 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:

118 Moby Thesaurus words for "substitution":
   agent, alienation, alternate, alternative, amends, analogy,
   atonement, autism, autistic thinking, avoidance mechanism, backup,
   balancing, blame-shifting, change, changeling, changing,
   commutation, comparison, compensation, copy, counteraction,
   counterbalancing, counterfeit, decompensation, defense mechanism,
   deputy, dereism, dereistic thinking, displacement, dissociation,
   double, dummy, emotional insulation, equal, equivalent, ersatz,
   escape, escape into fantasy, escape mechanism, escapism, exchange,
   expiation, fake, fantasizing, fantasy, fill-in, flight, ghost,
   ghostwriter, imitation, indemnification, indemnity, interchange,
   interchanging, isolation, lex talionis, locum tenens, makeshift,
   metaphor, metonymy, negativism, next best thing, offsetting,
   overcompensation, personnel, phony, pinch hitter, projection,
   proxy, psychotaxis, rationalization, recompense, rectification,
   redress, relief, reparation, repayment, replacement, replacing,
   representative, reserves, resistance, restitution, retaliation,
   revenge, ringer, satisfaction, second string, secondary, sign,
   sociological adjustive reactions, spares, stand-in, sub,
   sublimation, substituent, substitute, succedaneum, superseder,
   supplanter, supplanting, surrogate, swap, swapping, switch,
   switching, symbol, synecdoche, third string, token, understudy,
   utility player, vicar, vice-president, vice-regent,
   wish-fulfillment fantasy, wishful thinking, withdrawal



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

SUBSTITUTION, civil law. In the law of devises, it is the putting of one
person in the place of another, so that he may, in default of ability in the
former, or after him, have the benefit of a devise or legacy.
     2. It is a species of subrogation made in two different ways; the first
is direct substitution, and the latter a trust or fidei commissary
substitution. The first or direct substitution, is merely the institution of
a second legatee, in case the first should be either incapable or unwilling
to accept the legacy; for example, if a testator should give to Peter his
estate, but in case he cannot legally receive it, or he willfully refuses
it, then I give it to Paul; this is a direct substitution. Fidei commissary
substitution is that which takes place when the person substituted is not to
receive the legacy until after the first legatee, and consequently must
receive the thing bequeathed from the hands of the latter for example, I
institute Peter my heir, and I request that at his death he shall deliver my
succession to Paul. Merl. Repert. h.t.; 5 Toull. 14.



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

SUBSTITUTION, chancery practice. This takes place in a case where a creditor
has a lien on two different parcels of land, and another creditor has a
subsequent lien on one only of the parcels, and the prior creditor elects to
have his whole demand out of the parcel of land on which the subsequent
creditor takes his lien; the latter is entitled, by way of substitution, to
have the prior lien assigned to him for his benefit. 1 Johns. Ch. R. 409; 2
Hawk's Rep. 623; 2 Mason, R. 342. And in a case where a bond creditor exacts
the whole of the debt from one of the sureties, that surety is entitled to
be substituted in his place, and to a cession of his rights and securities,
as if be were a purchaser, either against the principal or his co-sureties.
Id. 413; 1 Paige's R. 185; 7 John. Ch. Rep. 211; 10 Watts, R. 148.
     2. A surety on paying the debt is entitled to stand in the place of the
creditor and to be subrogated to all his rights against the principal. 2
Johns. Ch. R. 454. 4 Johns. Ch. R. 123; 1 Edw. R. 164; 7 John. R. 584; 3
Paige's R. 117; 2 Call, R. 125; 2 Yerg. R. 346; 1 Gill & John. 346; 6 Rand.
R. 98,; 8 Watts, R. 384. In Pennsylvania it is provided by act of assembly,
that in all cases where a constable shall be entrusted with the execution of
any process for the collection of money, and by neglect of duty shall fail
to collect the same, by means whereof the bail or security of such constable
shall be compelled to pay the amount of any judgment shall vest in the
person paying, as aforesaid, the equitable interest in such judgment, and
the amount due upon any such judgment may be collected in the name of the
plaintiff for the use of such person. Pamphlet Laws, 1828-29, p. 370. Vide 2
Binn. R. 382, and Subrogation.