Holy Orders

Holy Orders is the Sacrament by which new priests succeed the Apostles. Through this Sacrament, a man is brought to the clerical state as a deacon, priest, or bishop, though in the past, there were other lower degrees of order, which have since been consolidated into the three we know today. Like Baptism and Confirmation, Orders confers a permanent ontological change upon the recipient, and is thus not repeatable, however, it can be, and typically is, conferred partially, which allows for it to be conferred more completely at a later time (a layman is ordained a deacon, a deacon a priest, a priest a bishop, though these are not separate Sacraments, but differing shares in the one priesthood).

From the Catholic Encyclopaedia

Order is the appropriate disposition of things equal and unequal, by giving each its proper place (St. Augustine, City of God XIX.13). Order primarily means a relation. It is used to designate that on which the relation is founded and thus generally means rank (St. Thomas, Supplement 34.2 ad 4um). In this sense it was applied to clergy and laity (St. Jerome, "In Isaiam", XIX, 18; St. Gregory the Great, "Moral.", XXXII, xx). The meaning was restricted later to thehierarchy as a whole or to the various ranks of the clergy. Tertullian and some early writers had already used the word in that sense, but generally with a qualifying adjective (Tertullian, Exhortation to Chastity 7, ordo sacerdotalis, ordo ecclesiasticus; St. Gregory of Tours, "Vit. patr.", X, i, ordo clericorum). Order is used to signify not only the particular rank or general status of the clergy, but also the outward action by which they are raised to that status, and thus stands for ordination. It also indicates what differentiates laity from clergy or the various ranks of the clergy, and thus means spiritual power. The Sacrament of Order is the sacrament by which grace and spiritual power for the discharge ofecclesiastical offices are conferred.