The title protosyngellos (in Greek, πρωτοσυγκελλος; also spelled protosynkellos or protosyncellus) is an honorific given by the bishop to a high-ranking cleric in a diocese, usually a priest. The protosyngellos is often the most senior cleric in the diocese after the bishop. It is sometimes given simply to whomever has the most seniority, but it may also be an office, often translated into English as chancellor, chief bishop's chaplain or episcopal vicar.
The term protosyngellos literally means "first cellmate." A syngellos is simply the cellmate of a monastic, while the cellmate of the bishop (who may often be a monastic) is styled the protos among the syngelloi. In the early use of this title, it referred to a monastic who lived with the bishop and served him.
Through history, however, as with many such titles from the Eastern Roman Empire, they shifted from referring explicitly to literal roles and became associated with clerical rank, sometimes including an office. In the modern era, the protosyngellos may often act as a chancellor to the bishop or may simply be the highest in seniority.