What is Brief?

Legal Definition
A brief (Old French from Latin "brevis", short) is a written legal document used in various legal adversarial systems that is presented to a court arguing why one party to a particular case should prevail.

In England and Wales, the phrase refers to the papers given to a barrister when they are instructed.
-- Wikipedia
Legal Definition
Eccl. law. The name of a kind of papal rescript. Briefs are writings sealed with wax, and differ in this respect from bulls, (q. v.) which are scaled with lead. They are so called, because they usually are short compendious writings. Ayl. Parerg. 132. See Breve.
-- Bouviers Law Dictionary
Legal Definition
Practice. An abridged statement of a party's case.

2. It should contain : 1st. A statement of the names of the parties, and of their residence and occupation, the character in which they sue and are sued, and wherefore they prosecute or resist the action. 2d. An abridgment of all the pleadings. 3d. A regular, chronological, and methodical statement of the facts in plain common language. 4th. A summary of the points or questions in issue, and of the proof which is to support such issues, mentioning specially the names of the witnesses by which the facts are to be proved, or if there be written evidence, an abstract of such evidence. 5th. The personal character of the witnesses should be mentioned; whether the moral character is good or bad, whether they are naturally timid or over-zealous, whether firm or wavering. 6th. If known, the evidence of the opposite party, and such facts as are adapted to oppose, confute, or repel it. Perspicuity and conciseness are the most desirable qualities of a brief, but when the facts are material they cannot be too numerous when the argument is pertinent and weighty, it cannot be too extended.

3. Brief is also used in the sense of breve. (q. v.)
-- Bouviers Law Dictionary
Legal Definition
In general. A written document; a letter; a writing lu the form of a letter. A summary, abstract or epitome. A condensed statement of some larger document or of a series of papers, facts or propositions. An epitome or condensed summary of the facts and circumstances, or propositions of law, constituting the case proposed to be set up by either party to an action about to be tried or argued. In English practice. A document prepared by the attorney, and given to the barrister, before the trial of a cause, for the lu-struction and guidance of the latter. It contains, in general, all the information necessary to enable the barrister to successfully conduct their client's case in court, such as a statement of the facts, a summary of the plcadings, the names of the witnesses, and an outllne of the evidence expected from them, and any suggestions arising out of the peculiarities of the case. In American practice. A written or printed document, prepared by counsel to serve as the basis for an argument upon a cause in an appellate court, and usually filed for the information of the court. It embodies the points of law which the counsel desires to establish, together with the arguments and authorities upon which he rests his contention. A brief, within a rule of court requiring counsel to furnish briefs, before argument, implies some kind of statement of the case for the information of the court. Gardner v. Stover, 43 Ind. S56. In Scotch law. Brief is used in the sense of "writ," and this seems to be the sense in which the word is used in very many of the ancient writers. In ecclesiastical law. A papal rescript sealed with wax. See Bull.
See also
-- Black's Law Dictionary
Legal Definition
An outline of the case of one of the parties; a written argument prepared for the court; to prepare a brief. See 43 Ind. 356.
-- Ballentine's Law Dictionary
Legal Definition
Written arguments submitted to the court.