Episcopal ordination - fullness of the sacrament of Holy Orders
"Amongst those various offices which have been exercised in the Church from the earliest times the chief place, according to the witness of tradition, is held by the function of those who, through their appointment to the dignity and responsibility of bishop, and in virtue consequently of the unbroken succession going back to the beginning, are regarded as transmitters of the apostolic line."
Source: Catechism of the Catholic Church (Vatican.va)