Overclass is a recent and pejorative term for the most powerful group in a social hierarchy. Users of the term generally imply excessive and unjust privilege and exploitation of the rest of society.
The word is fairly recent: the Oxford English Dictionary included it only in December 2004. But it has been in use since at least 1995. Some writers compare it to the more familiar underclass:
We now have a quite new phenomenon in the history of the republic: two radically isolated sectors of the population, the underclass and the overclass. Both are in an adversarial posture toward the great majority of Americans, the overclass by virtue of ambition and unbounded self-esteem, the underclass by virtue of social incompetence and anomie. Between the two there is a fearful symmetry on many scores, but their service to each other is far from equal.
The influence of the actions by the overclass have been rigorously studied, particularly with regards to notions of intersections between the overclass and specific races. Most notable of these racial overclasses is the NEWBO, or NEW Black Overclass in America.
Perhaps the most commonly agreed-upon "overclass" consists of leaders in international business, finance and the arms trade.