Objection 1. It would seem that Order is not a sacrament. For a sacrament, according to Hugh of St. Victor (De Sacram. i) "is a material element." Now Order denotes nothing of the kind, but rather relation or power; since Order is a part of power according to Isidore. Therefore it is not a sacrament.
Objection 2. Further, the sacraments do not concern the Church triumphant. Yet Order is there, as in the angels. Therefore it is not a sacrament.
Objection 3. Further, just as spiritual authority, which is Order, is given by means of consecration, so is secular authority, since kings also are anointed, as stated above (19, 3, ad 2). But the kingly power is not a sacrament. Therefore neither is order of which we speak now.
On the contrary, It is mentioned by all among the seven sacraments of the Church.
Further, "the cause of a thing being such, is still more so." Now Order is the cause of man being the dispenser of the other sacraments. Therefore Order has more reason for being a sacrament than the others.
I answer that, As stated above (29, 1; III, 60), a sacrament is nothing else than a sanctification conferred on man with some outward sign. Wherefore, since by receiving orders a consecration is conferred on man by visible signs, it is clear that Order is a sacrament.
Reply to Objection 1. Although Order does not by its name express a material element, it is not conferred without some material element.
Reply to Objection 2. Power must needs be proportionate to the purpose for which it is intended. Now the communication of divine things, which is the purpose for which spiritual power is given, is not effected among the angels by means of sensible signs, as is the case among men. Hence the spiritual power that is Order is not bestowed on the angels by visible signs, as on men. Wherefore Order is a sacrament among men, but not among angels.
Reply to Objection 3. Not every blessing or consecration given to men is a sacrament, for both monks and abbots are blessed, and yet such blessings are not sacraments, and in like manner neither is the anointing of a king; because by such blessings men are not ordained to the dispensing of the divine sacraments, as by the blessing of Order. Hence the comparison fails.
Source: Summa Theologica (New Advent)