What is Presbyterium?

Legal Definition
Presbyterium is a modern term used in the Catholic Church and Eastern Catholic Churches after the Second Vatican Council in reference to a college of priests, in active ministry, of an individual particular church such as a diocese or eparchy. The body, in union with their bishop as a collective, is a symbol of the collaborative and collegial nature of their sacerdotal ministry as inspired by the reforms made during the Second Vatican Council.

The presbyterium is most visible during the ordination of new priests and bishops and the Mass of the Chrism: the Holy Thursday Mass where the blessing of the oils used in the sacraments of Baptism, Confirmation, Anointing of the Sick, and Holy Orders takes place. They are also visible during other special liturgical functions such as the wake and burial of their bishop.

Liturgically, within Catholicism, the presbyterium is the area of the church in which the clergy functions. It is more commonly called the sanctuary.
-- Wikipedia
Legal Definition
That part of the church where divine offices are performed; formerly applied to the choir or chancel, because it was the place appropriated to the bishop, priest, and other clergy, while the laity were confined to the body of the church. Jacob.
-- Black's Law Dictionary