What is Holy Orders?

Legal Definition
In the Christian churches, Holy Orders are ordained ministries such as bishop, priest or deacon. In the Roman Catholic (Latin: sacri ordines), Eastern Catholic, Eastern Orthodox (ιερωσύνη [hierōsynē], ιεράτευμα [hierateuma], Священство [Svyashchenstvo]), Oriental Orthodox, Anglican, Assyrian, Old Catholic, Independent Catholic and some Lutheran churches, holy orders is comprised is the three ministerial orders of bishop, priest and deacon, or the sacrament or rite by which candidates are ordained to those orders. Except for Lutherans and some Anglicans, these churches regard ordination as a sacrament (the sacramentum ordinis). The Anglo-Catholic tradition within Anglicanism identifies more with the Roman Catholic position about the sacramental nature of ordination.

Denominations have varied conceptions of holy orders. In the Anglican churches and some Lutheran churches the traditional orders of bishop, priest and deacon are bestowed using ordination rites. The extent to which ordination is considered sacramental in these traditions has, however, been a matter of some internal dispute. Many other denominations do not consider ministry as being sacramental in nature and would not think of it in terms of "holy orders" as such. Historically, the word "order" (Latin ordo) designated an established civil body or corporation with a hierarchy, and ordinatio meant legal incorporation into an ordo. The word "holy" refers to the Church. In context, therefore, a holy order is set apart for ministry in the Church. Other positions, such as pope, patriarch, cardinal, monsignor, archbishop, archimandrite, archpriest, protopresbyter, hieromonk, protodeacon and archdeacon, are not sacramental orders but particular ministry positions.
-- Wikipedia
Legal Definition
In ecclesiastical law. The orders of bishops, (including archbishops,) priests, and deacons in the Church of England. The Roman canonists had the orders of bishop, (in which the pope and archbishops were included,) priest, deacon, subdeacon, psalmist, acolyte, exorcist, reader, ostiarius. 3 Steph. Comm. 55, and note a.
-- Black's Law Dictionary