noun
idiosyncrasy refers to any personal mannerism or peculiarity and connotes strong individuality the idiosyncrasies of a writer's style; eccentricity implies considerable deviation from what is normal or customary and connotes whimsicality or even mental aberration his eccentricity of wearing overshoes in the summer
See idiosyncrasy in Webster's New World Roget's A-Z Thesaurus II
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