Holy Orders is the sacrament through which the mission entrusted by Christ to his apostles continues to be exercised in the Church until the end of time; thus it is the sacrament of apostolic ministry. It includes three degrees: Episcopate, Presbyterate and Diaconate.
The word Order in Roman antiquity designated an established civil body, especially a governing body. Ordinatio mean incorporating into an Ordo. In the Church there are established bodies which Tradition, not without a basis in Sacred Scripture, has since ancient times called “taxeis (Greek) or “ordines”. And so the liturgy speaks of the ordo episcoporum, the ordo presbyterorum,and the ordo diaconorum. Other groups also receive this name of ordo: Catechumens, virgins, spouses, widows, and so on.
Integration into one of thebodies in the Church was accomplished by a rite called ordination, a religious and liturgical act which was a consecration, a blessing or a sacrament. Today the word ordination is reserved for the sacramental act which integrates a man into the order of bishops, presbyters or deacons and goes beyond a simple election, designation, delegation or institution by the community, for it confers an indelible nature and a gift of the Holy Spirit that permits the exercise of a “sacred power” (sacra potestas) which can only come from Christ himself through his Church.