What is Allegation?

Legal Definition
In law, an allegation (also called adduction) is a claim of a fact by a party in a pleading, charge, or defense. Until they can be proved, allegations remain merely assertions.

There are also marital allegations: marriage bonds and allegations exist for couples who applied to marry by licence. They do not exist for couples who married by banns. The marriage allegation was the document in which the couple alleged (or frequently just the groom alleged on behalf of both of them) that there were no impediments to the marriage.

Generally, in a civil complaint, a plaintiff alleges facts sufficient to establish all the elements of the claim and thus states a cause of action. The plaintiff must then carry the burden of proof and the burden of persuasion in order to succeed in the lawsuit.

A defendant can allege affirmative defenses in its answer to the complaint.

Other allegations are required in a pleading to establish the correct jurisdiction, personal jurisdiction and subject matter jurisdiction.
-- Wikipedia
Legal Definition
A claim of fact not yet proven to be true. In a lawsuit, a party puts forth their allegations in a complaint, indictment or affirmative defense, and then uses evidence at trial to attempt to prove their truth.
Legal Definition
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.
-- Bouviers Law Dictionary
Legal Definition
The assertion, declaration or statement of a party to an action, made in a pleading, setting out what he expects to prove. A material allegation in a pleading is one essential to the claim or defense, and which could not be stricken from the pleading without leaving it insufficient. Code Civil Proc. Cal. ยง 463. In ecclesiastical law. The statement Of the facts intended to be relied on in support of the contested suit. In English ecclesiastical practice the word seems to designate the pleading as a whole; the three pleadings are known as the allegations ; and the defendant's plea is distinguished as the defensive, or sometimes the responsive, allegation, and the complainant's reply as the rejoining allegation. See Allegation of faculties.
-- Black's Law Dictionary
Legal Definition
Common law. The assertion, declaration or statement of a party of what he can prove.
-- Bouviers Law Dictionary
Legal Definition
An assertion; a pleading.
-- Ballentine's Law Dictionary