"form of government in which supreme power is vested in a small exclusive class," 1570s, from French oligarchie (14c.), from Latinized form of Greek oligarkhia "government by the few," from stem of oligos "few, small, little" (a word of uncertain origin) + -arkhia, from arkhein "to rule" (see archon). An earlier form of the word in English was oligracie (c. 1500, from Old French).
Aristotle, after some preliminary remarks, concludes by defining a democracy to be, when the freemen and those not the rich, being the majority, possess the sovereign power; and an oligarchy, when the rich and those of noble birth, being few, are in possession of the sovereign power. This definition of an oligarchy necessarily implies that the majority are excluded from participating in the sovereign power. It might be inferred, on the other hand, that in this definition of a democracy the few are excluded from the sovereign power: and such in this passage should be the meaning of the author, if he is consistent with himself. ["Political Dictionary," London 1845]