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.
Eccentricity Facts for Kids. Kiddle Encyclopedia.