Objection 1. It would seem that the episcopate is an Order. First of all, because Dionysius (Eccl. Hier. v) assigns these three orders to the ecclesiastical hierarchy, the bishop, the priest, and the minister. In the text also (Sent. iv, D, 24) it is stated that the episcopal Order is fourfold.
Objection 2. Further, Order is nothing else but a degree of power in the dispensing of spiritual things. Now bishops can dispense certain sacraments which priests cannot dispense, namely Confirmation and Order. Therefore the episcopate is an Order.
Objection 3. Further, in the Church there is no spiritual power other than of Order or jurisdiction. But things pertaining to the episcopal power are not matters of jurisdiction, else they might be committed to one who is not a bishop, which is false. Therefore they belong to the power of Order. Therefore the bishop has an Order which a simple priest has not; and thus the episcopate is an Order.
On the contrary, One Order does not depend on a preceding order as regards the validity of the sacrament. But the episcopal power depends on the priestly power, since no one can receive the episcopal power unless he have previously the priestly power. Therefore the episcopate is not an Order.
Further, the greater Orders are not conferred except on Saturdays [The four Ember Saturdays]. But the episcopal power is bestowed on Sundays [Dist. lxxv, can. Ordinationes]. Therefore it is not an Order.
I answer that, Order may be understood in two ways. In one way as a sacrament, and thus, as already stated (37, 2,4), every Order is directed to the sacrament of the Eucharist. Wherefore since the bishop has not a higher power than the priest, in this respect the episcopate is not an Order. In another way Order may be considered as an office in relation to certain sacred actions: and thus since in hierarchical actions a bishop has in relation to the mystical body a higher power than the priest, the episcopate is an Order. It is in this sense that the authorities quoted speak.
Hence the Reply to the First Objection is clear.
Reply to Objection 2. Order considered as a sacrament which imprints a character is specially directed to the sacrament of the Eucharist, in which Christ Himself is contained, because by a character we are made like to Christ Himself [Cf. III, 63, 3]. Hence although at his promotion a bishop receives a spiritual power in respect of certain sacraments, this power nevertheless has not the nature of a character. For this reason the episcopate is not an Order, in the sense in which an Order is a sacrament.
Reply to Objection 3. The episcopal power is one not only of jurisdiction but also of Order, as stated above, taking Order in the sense in which it is generally understood.
Source: Summa Theologica (New Advent)