In ancient times bishops, as rulers of cities and provinces, especially in the Papal States, were called rectors, as were administrators of the patrimony of the Church (e.g. rector Siciliae). The Latin term rector was used by Pope Gregory I in Regula Pastoralis as equivalent to the Latin term pastor (shepherd).
In the Roman Catholic Church, a rector is a person who holds the office of presiding over an ecclesiastical institution. The institution may be a particular building—like a church (called his rectory church) or shrine—or it may be an organization, such as a parish, a mission or quasi-parish, a seminary or house of studies, a university, a hospital or a community of clerics or religious.
If a rector appointed as his employee someone to perform the duties of his office, i.e. to act for him "vicariously", that employee was termed his vicar. Thus, the tithes of a parish are the legal property of the person who holds the office of rector, and are not the property of his vicar, who is not an office-holder but a mere employee, remunerated by a stipend, i.e. a salary, payable by his employer the rector. Thus, a parish vicar is the vicarious agent of his rector, whilst, higher up the scale, the Pope is called the Vicar of Christ, acting vicariously for the ultimate superior in the ecclesiastical hierarchy.
However, these are not the only officials who exercise their functions using the title of rector. Since the term rector refers to the function of the particular office, a number of officials are not referred to as rectors even though they are rectors in actual practice. The diocesan bishop, for instance, is himself a rector, since he presides over both an ecclesiastical organization (the diocese) and an ecclesiastical building (his cathedral). In many dioceses, the bishop delegates the day-to-day operation of the cathedral to a priest, who is often incorrectly called a rector but whose specific title is plebanus or "people's pastor", especially if the cathedral operates as a parish church. Therefore, because a priest is designated head of a cathedral parish, he cannot be both rector and pastor, as a rector cannot canonically hold title over a parish (c.556).
As a further example, the pastor of a parish (parochus) is pastor (not rector) over both his parish and the parish church. Finally, a president of a Catholic university is rector over the university and, if a priest, often the rector of any church that the university may operate, on the basis that it is not a canonical establishment of a parish (c. 557 §3).