What term is used to refer to the officiant who celebrates the mass in the Roman Catholic Church?

The term that refers to the officiant who celebrates the mass in the Roman Catholic Church is "celebrant." This term specifically designates the priest or bishop who leads the Eucharistic celebration, which is central to Catholic worship. The celebrant is responsible for the liturgical prayers, the consecration of the bread and wine, and guiding the congregation through the various parts of the mass.

In contrast, other terms in the question refer to different contexts within the church. A chaplain typically serves in a specific institution or setting, such as a hospital or military unit, providing spiritual guidance and support to individuals there. Chancel refers to the area around the altar in a church where the clergy and singers perform, but it is not a term for the person officiating the mass. A columbarium is a structure designed to hold urns containing ashes of the deceased and does not pertain to the celebration of the mass. Thus, "celebrant" is the correct term for the role being described.