The Sacrament of Holy Orders is the continuation of Christ's priesthood, which He bestowed upon His Apostles; thus, the Catechism of the Catholic Church refers to the Sacrament of Holy Orders as "the sacrament of apostolic ministry."
"Ordination" comes from the Latin word ordinatio, which means to incorporate someone into an order. In the Sacrament of Holy Orders, a man is incorporated into the priesthood of Christ, at one of three levels: the episcopate, the priesthood, or the diaconate.