Oligarchy is a term deriving from the Greek words 'ὀλίγος', meaning "few", and 'ἄρχω', meaning "rule". Broadly speaking, it refers to a situation in which political power resides in a particular minority group, as opposed to a single individual or a broader base. This label applies irrespective of the type of group wielding political power, for which there are more specific terms; "oligarchy" thus covers a rather wide range of possible political arrangements.
In practice, many governments that begin in some other form mutate into a type of oligarchy, even though they may retain the trappings of their origins. This usually results from the longtime association and alliance-building of certain individuals or factions in government. While oligarchic rule is not necessarily harmful in and of itself, the concentration of power can be conducive to exploitation or exclusion of the more numerous segments of the populace by the oligarchs (or to the perception by the populace that such activities are occurring, regardless of whether they are or not). Oligarchies are predominantly authoritarian dictatorships.
Drawing from the Greek definition of the word, it is generally accepted that the Ancient Greek city-state of Sparta were among the first to practise an oligarchy in an organised government. Over Classical Greek history, the oligarchic government of Sparta would clash with the democratic government of Athens, eventually culminating in the Second Peloponnesian War, in which Athens was defeated and a Spartan oligarchy, the 'Thirty Tyrants', ruled Athens until democracy was ultimately restored in 403 BC.