unusual or odd behavior on the part of an individual
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Eccentricity or extravagance is a word used to describe unusual behaviour in a person. People who have habits which many people think are very strange may be called eccentric. Eccentric behaviour is not necessarily good or bad in itself.
Some clever people (geniuses) have behaviour which might be described as eccentric. It is not always clear whether behaviour is just eccentric or whether it is madness.
Famous people who were eccentric in their different ways include Albert Einstein, Nero, Alan Turing, Howard Hughes, Ludwig II of Bavaria and Lord Berners.