The term 'major orders' was a part of the clerical terminology of the
Roman Catholic Church until shortly after the
Second Vatican Council, when the use widely disappeared due to reform of the clerical structure. During the
Counter-reformation, the
Council of Trent issued a decree outlining the
orders of the
clergy. The first four, the
minor orders, have various
liturgical functions and were conferred upon seminarians studying for the priesthood. The major orders were the final ones, and the ministers in those orders were:
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Subdeacon,
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Deacon,
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Priest, and
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Bishop
There is disagreement as to whether Council of Trent defined seven or eight orders. Chapter II of the decree talks about seven orders, omitting bishops, which would indicate that bishops are merely super-priests and not a separate order. However, chapter IV talks of bishops, distinguishes them from the other ecclesiastical degrees of ministers, and discusses their ordination, indicating that they are a separate order from the presbyteriate. Due to the debate, the ministers in the orders are presented here, without making a claim on three or four orders.

Priestly ordination is part of major orders.
A man could be admitted to the major orders only after receiving the minor orders. The vestments common to all those in major orders are the
maniple, which was worn in liturgical functions until the
Second Vatican Council. Each order has a distinctive outer vestment for
Mass, with the subdeacon wearing the
tunicle, the deacon wearing the
dalmatic, and priests and bishops wearing the
chasuble. Deacons, priests, and bishops also wear the
stole, the common garment of
Holy Orders.
The reason that these orders were considered "major" was that, with
ordination to the subdiaconate, both the recitation of the
Liturgy of the Hours and perpetual
celibacy became mandatory. Those in minor orders do not have this obligation. Thus, e.g., an
acolyte did not have to pray the Divine Office and could marry without
Papal dispensation should he leave the seminary. These lesser orders along with the subdiaconate were seen simply as liturgical functions, which could be changed on the authority of the
Church. This is exactly what happened in
1972, when
Pope Paul VI suppressed the subdiaconate and changed the minor orders to
lay ministries. The major and minor orders, including the subdiaconate, however remained in use at conservative
Traditionalist Roman Catholic seminaries.
References
Council of Trent Decree on the Sacrament of Order