The Collaborative International Dictionary of English v.0.48:

Allegation \Al`le*ga"tion\, n. [L. allegatio, fr. allegare,
   allegatum, to send a message, cite; later, to free by giving
   reasons; ad + legare to send, commission. Cf. Allege and
   Adlegation.]
   1. The act of alleging or positively asserting.
      [1913 Webster]

   2. That which is alleged, asserted, or declared; positive
      assertion; formal averment
      [1913 Webster]

            I thought their allegation but reasonable. --Steele.
      [1913 Webster]

   3. (Law) A statement by a party of what he undertakes to
      prove, -- usually applied to each separate averment; the
      charge or matter undertaken to be proved.
      [1913 Webster]

WordNet (r) 3.0 (2006):

allegation
    n 1: (law) a formal accusation against somebody (often in a
         court of law); "an allegation of malpractice"
    2: statements affirming or denying certain matters of fact that
       you are prepared to prove [syn: allegation, allegement]

Moby Thesaurus II by Grady Ward, 1.0:

112 Moby Thesaurus words for "allegation":
   Parthian shot, accusal, accusation, accusing, address, admission,
   affidavit, affirmance, affirmation, allegement, announcement,
   annunciation, answer, apostrophe, arraignment, assertion,
   asseveration, attest, attestation, averment, avouchment, avowal,
   bill, bill of complaint, bill of particulars, blame,
   bringing of charges, bringing to book, charge, claim, comment,
   complaint, compurgation, conclusion, count, crack, creed,
   declaration, delation, denouncement, denunciation, deposition,
   dictum, disclosure, enunciation, exclamation, expression, greeting,
   impeachment, implication, imputation, indictment, information,
   innuendo, insinuation, instrument in proof, interjection,
   ipse dixit, lawsuit, laying of charges, legal evidence, libel,
   manifesto, mention, narratio, nolle prosequi, nonsuit, note,
   observation, phrase, plaint, position, position paper,
   positive declaration, predicate, predication, proclamation,
   profession, pronouncement, proposition, prosecution, protest,
   protestation, question, reflection, remark, reproach, say, say-so,
   saying, sentence, stance, stand, statement, statement of facts,
   subjoinder, suit, sworn evidence, sworn statement, sworn testimony,
   taxing, testimonial, testimonium, testimony, thought, true bill,
   unspoken accusation, utterance, veiled accusation, vouch, witness,
   word



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

ALLEGATION, common law. The assertion, declaration or statement of a party
of what he can prove.



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

ALLEGATION, civil law. The citation or reference to a voucher  to support a
proposition. Dict. de jurisp.; Encyclopedie, mot Allegation; 1 Brown's Civ.
Law, 473, n.

ALLEGATION OF FACULTIES When a suit is instituted in the English
ecclesiastical courts, in order to obtain alimony, before it is allowed, an
allegation must be made on the part of the wife, stating the property of the
husband. This allegation is called an allegation of faculties. Shelf. on
Mar. and Div. 587.



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

ALLEGATION, English ecclesiastical law. According to the practice of the
prerogative court, the facts intended to be relied on in support of the
contested suit are set forth in the plea, which is termed an allegation;
this is submitted to the inspection of the counsel of the adverse party,
and, if it appear to them objectionable in form or substance, they oppose
the admission of it. If the opposition goes to the substance of the
allegation, and is held to be well founded, the court rejects it; by which
mode of proceeding the suit is terminated without, going into any proof of
the facts. 1 Phil. 1, n.; 1 Eccl. Rep. ll, n. S. C. See 1 Brown's Civ. Law,
472, 3, n.