What is the term for a geographical grouping of parishes under a bishop in the Roman Catholic faith?

The term for a geographical grouping of parishes under a bishop in the Roman Catholic faith is "diocese." A diocese serves as the foundational administrative unit within the Church, enabling effective oversight and pastoral care for the congregations within its boundaries. The bishop, as the spiritual leader, is responsible for the administration of the diocese and the spiritual guidance of its members.

An archdiocese represents an elevated status of a diocese, typically overseeing multiple dioceses but is not the basic unit. A bishopric generally refers to the office or term of a bishop but does not specifically denote the geographical arrangement of parishes. A congregation usually refers to a group of worshippers or a specific community of faith rather than the organizational structure defined by geographical boundaries. Thus, "diocese" accurately describes the intended concept in the question.