Webster's Dictionary 
(1): (n.) An abridgment; a compend; an epitome; a brief account or summary.
(2): (n.) A book containing the daily public or canonical prayers of the Roman Catholic or of the Greek Church for the seven canonical hours, namely, matins and lauds, the first, third, sixth, and ninth hours, vespers, and compline; - distinguished from the missal.
Charles Buck Theological Dictionary 
The book containing the daily service of the church of Rome.
Cyclopedia of Biblical, Theological and Ecclesiastical Literature 
(Breviarium), the daily service-book of the priests of the Roman Church. It was originally called the Cursus. The origin of the name Breviary is not very certain; the most likely derivation is from brevis, denoting that the service-book called Breviary was originally an abridged one, as contrasted with Plenariunm offcium. It contains prayers for Matins, Lauds (3 A.M.), Prime (6 A.M.), Tierce, Sext (all before 12 M.), Nones, Vespers (P.M.), and Compline (before going to sleep). Nocturn was properly a night service. The custom of saying prayers at these different hours is very ancient. The author of the Apostolical Constitutions directs that prayer should be made "Mane, Tertia, Sexta, Nona, Vespere, atq. ad galli canturn" (Const. 8). Basil speaks of seven distinct appointed hours of prayer, and Tertullian mentions Tierce, Sext, and None, which he calls apostolical hours of prayer (De Jejuniis, c. 11). Cyprian also speaks of "leorce antiquitus observatoe orandi" (De Orat. Domin.).
Gregory VII (1074) compiled the first Breviary which came into general use. As most churches possessed compilations of the offices severally in use among them, there are various Breviaries differing one from another. Attempts have been made to amend the Breviary at different times, and so there are many differences among them in different dioceses. That of Rome, however (Breviarium Romanum), is most widely circulated, and of late has been introduced into many dioceses which long resisted it. It consists of four parts: the Psalterium, or psalms for the canonical hours; Proprium de Tempore, for Advent and other festivals commemorative of Christ; Proprium de Sanctis, for saints' days; Commune Sanctorum, for festivals to which no special hours of prayer are assigned. Besides psalms, lessons, homilies, and prayers, it contains many foolish legends and absurd stories about saints, which are cause of scandal to the better sort of Romanists. In fact, a proverb in use among scholars of the Roman Church says of a liar, Mentitur sicut secundus nocturnus. As to the duty of using the Breviary, it was at first enjoined on both clergy and laity; but, lby degrees, the obligation was reduced to the clergy only, who are required, under penalty of mortal sin and ecclesiastical censures, to recite it at home when they can not attend in public (Conc. Trid. sess. 24:cap. 12). In the fourteenth century there was a reserve granted in favor of bishops, who were allowed, on particular occasions, to pass three days without rehearsing the Breviary. One of the best editions of the Breciarium Romanum is that of Mechlin, 1886 (4 vols. 12mo). For a full account of its history and contents, see Lewis, Bible, Missal, and Breviary (Edinb. 1853, 2 vols. 8vo).
The Breviary of the Greeks, which they call by the name ῾Ωρολόγιον (Horologiiu), Dial, is the same in almost all the churches and monasteries which follow the Greek rites. The Greeks divide the Psalter into twenty parts, called Καθίσματα (Sedilia), Seats, because they are a kind of pauses or rests. In general, the Greek Breviary consists of two parts, the one containing the office for the evening, the other that for the morning, divided into matins, lauds, first, third, sixth, ninth, vespers, and the compline; that is, of seven different hours, on account of that saying of David, "Seven times in the day will I praise Thee." The compline is the last office at night, by which the work of the day is complete (Fr. compline, Lat. completinum).-Bergier, s.v. Office Divin.; Bingham, Orig. Eccl. bk. 13:ch. 9: § 8; Procter, On Common Prayer, p. 11. (See Liturgy).
The Nuttall Encyclopedia 
A book containing the daily services in the Roman Catholic Church and corresponding to the English Prayer-Book; differs from the "Missal," which gives the services connected with the celebration of the Eucharist, and the "Pontifical," which gives those for special occasions.