The Collaborative International Dictionary of English v.0.48:
Oligarch \Ol`i*garch\, n.
A member of an oligarchy; one of the rulers in an
WordNet (r) 3.0 (2006):
n 1: one of the rulers in an oligarchy
Bouvier's Law Dictionary, Revised 6th Ed (1856):
OLIGARCHY. This name is given to designate the power which a few citizens of
a state have usurped, which ought by the constitution to reside in the
people. Among the Romans the government degenerated several times into an
oligarchy; for example, under the decemvirs, when they became the only
magistrates in the commonwealth.