Amongst the Latins, the recitation of the Breviary was called the Office (officium), that is, the duty, the function, the office; because it is, par excellence, the duty, function and office of persons consecrated to God. This is the oldest and most universal name for the Breviary and its recitation. It was called, too, the Divine Office (officium divinum), because it has God for its principal object and is recited by persons consecrated to God. It is called the ecclesiastical office (officium ecclesiasticum), because it was instituted by the Church. Other names were, Opus Dei; Agenda; Pensum servitutis; Horae; Horae Canonicae.
Quigley, Edward J. (2012-05-17). The Divine Office (p. 2). Kindle Edition.