A person or (occasionally) an animal that has a quarter of a responsibility which is shared with another or others. Obsolete.

In Roman History: One of the duumviri, the general name given to pairs of co-equal magistrates and functionaries in Rome and in her coloniæ and…

transferred and figurative. One of four joint rulers, directors, or heads.

singular. A member of such a body.

A member of a quadrumvirate; one of a group of four leaders or commanders. In later use: a commander in Mussolini's Fascist government in Italy.

A member of a pentarchy; one of a group of five rulers.

one who shares rule with another.

One of a ruling body of ten.

One of a ruling body of twelve.