The Collaborative International Dictionary of English v.0.48:

Confirmation \Con`fir*ma"tion\, n. [F. confirmation, L.
   confirmatio.]
   1. The act of confirming or strengthening; the act of
      establishing, ratifying, or sanctioning; as, the
      confirmation of an appointment.
      [1913 Webster]

            Their blood is shed
            In confirmation of the noblest claim. --Cowper.
      [1913 Webster]

   2. That which confirms; that which gives new strength or
      assurance; as to a statement or belief; additional
      evidence; proof; convincing testimony.
      [1913 Webster]

            Trifles light as air
            Are to the jealous confirmations strong
            As proofs of holy writ.               --Shak.
      [1913 Webster]

   3. (Eccl.) A rite supplemental to baptism, by which a person
      is admitted, through the laying on of the hands of a
      bishop, to the full privileges of the church, as in the
      Roman Catholic, the Episcopal Church, etc.
      [1913 Webster]

            This ordinance is called confirmation, because they
            who duly receive it are confirmed or strengthened
            for the fulfillment of their Christian duties, by
            the grace therein bestowed upon them. --Hook.
      [1913 Webster]

   4. (Law) A conveyance by which a voidable estate is made sure
      and not voidable, or by which a particular estate is
      increased; a contract, express or implied, by which a
      person makes that firm and binding which was before
      voidable.
      [1913 Webster]

WordNet (r) 3.0 (2006):

confirmation
    n 1: additional proof that something that was believed (some
         fact or hypothesis or theory) is correct; "fossils provided
         further confirmation of the evolutionary theory" [syn:
         confirmation, verification, check, substantiation]
    2: information that confirms or verifies
    3: making something valid by formally ratifying or confirming
       it; "the ratification of the treaty"; "confirmation of the
       appointment" [syn: ratification, confirmation]
    4: a ceremony held in the synagogue (usually at Pentecost) to
       admit as adult members of the Jewish community young men and
       women who have successfully completed a course of study in
       Judaism
    5: a sacrament admitting a baptized person to full participation
       in the church

Moby Thesaurus II by Grady Ward, 1.0:

114 Moby Thesaurus words for "confirmation":
   John Hancock, OK, acceptance, affirmance, affirmation, agape,
   approbation, approval, ascertainment, asperges, aspersion,
   assurance, attestation, auricular confession, authentication,
   authorization, backing, backing up, baptism, bar mitzvah,
   bas mitzvah, bearing out, bolstering, buttressing, celebration,
   certification, check, checking, circumcision, circumstantiation,
   collation, comparative scrutiny, confession, corroboration,
   corroboratory evidence, countersignature, cross-check,
   deep-rootedness, deep-seatedness, determination, documentation,
   embedment, endorsement, ensuring, entrenchment, establishment,
   evidence, extreme unction, fixation, fixedness, fixity, fixture,
   fortification, go-ahead, green light, high celebration,
   holy orders, implantation, imprimatur, incense, infixion,
   inveteracy, invocation, invocation of saints, kiss of peace,
   lesser litany, litany, love feast, lustration, matrimony, nod,
   notarization, okay, pax, penance, permission, processional, proof,
   proving, proving out, ratification, reassurance, reassurement,
   reciting the rosary, reinforcement, rubber stamp, sanction, seal,
   seven sacraments, sigil, signature, signet, stabilization, stamp,
   stamp of approval, strengthening, subscription, substantiation,
   support, supporting evidence, telling of beads, testament,
   testimonial, the Eucharist, the confessional, the confessionary,
   the nod, undergirding, validation, verification, visa, vise,
   warrant, witness



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

CONFIRMATION, contracts, conveyancing. 1. A contract by which that which
was voidable, is made firm and unavoidable.
     2. A species of conveyance.
     2. - 1. When a contract has been entered into by a stranger without
authority, he in whose name it has been made may, by his own act, confirm
it; or if the contract be made by the party himself in an informal and
voidable manner, he may in a more formal manner confirm and render it valid;
and in that event it will take effect, as between the parties, from the
original making. To make a valid confirmation, the party must be apprised
of, his rights, and where there has been a fraud in the transaction, he must
be award of it, and intend to confirm his contract. Vide 1 Ball & Beatty,
353; 2 Scho. & Lef. 486; 12 Ves. 373; 1 Ves. Jr. 215; Newl. Contr. 496; 1
Atk. 301; 8 Watts. R. 280.
     3. - 2. Lord Coke defines a confirmation of an estate, to be "a
conveyance of an estate or right in esse, whereby a voidable estate is made
sure and unavoidable; or where a particular estate is increased."
     4. The first part of this definition may be illustrated by the
following case, put by Littleton, Sec. 516; where a person lets land to
another for the term of his life, who lets the same to another for forty
years, by force of which he is in possession; if the lessor for life
confirms the estate of the tenant for years by deed, and afterwards the
tenant for life dies, during the term; this deed will operate as a
confirmation of the term for years.. As to the latter branch of the
definition; whenever a confirmation operates by way of increasing the
estate, it is similar in every respect to a release that operates by way of
enlargement, for there must be privity of estate, and proper words of
limitation. The proper technical words of a confirmation are, ratify and
confirm; although it is usual and prudent to insert also the words given and
granted. Watk. Prin. Convey. chap. vii.
     5. A confirmation does not strengthen a void estate. Confirmatio est
nulla, ubi donum precedens est invalidum, et ubi donatio nulla est nec
valebit confirmatio. For confirmation may make a voidable or defeasible
estate good, but cannot operate on an estate void in law. Co. Litt. 295. The
canon law agrees with this rule, and hence the maxim, qui confirmat nihil
dat. Toull. Dr. Civ. Fr. liv. 3, t. 3, c. 6, n. 476. Vide Vin. Ab. h.t.;
Com. Dig. 11. t.; Ayliffe's Pand. *386; 1 Chit. Pr. 315; 3 Gill & John. 290;
3 Yerg. R. 405; Co. Litt. 295; Gilbert on Ten. 75; 1 Breese's R. 236; 9 Co.
142, a; 2 Bouv. Inst. n. 2067-9.
     6. An infant is said to confirm his acts performed during infancy,
when, after coming to full age, be expressly approves of them, or does acts
from which such confirmation way be implied. Sec Ratification.