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1560s, from Middle French aristocracie (Modern French aristocratie), from Late Latin aristocratia, from Greek aristokratia "government or rule of the best," from aristos "best" (originally "most fitting," from PIE *ar-isto-, superlative form of *ar- "to fit together;" see arm (n.1)) + kratos "rule, power" (see -cracy).
At first in a literal sense of "government by those who are the best citizens;" meaning "rule by a privileged class" (best-born or best-favored by fortune) is from 1570s and became paramount 17c. Hence, the meaning "patrician order" (1650s). In early use contrasted with monarchy; after French and American revolutions, with democracy.
A privileged, primarily hereditary ruling class, or a form of government controlled by such an elite.
Note: Traditionally, the disproportionate concentration of wealth, social status, and political influence in the aristocracy has been resented by the middle class and lower class.