an unusual habit or way of behaving
idiosyncrasy - quirk
If a person has an ‘idiosyncrasy’, they have a quirk or characteristic peculiarity that sets them apart from others. The word often appears in its plural form and is generally used in association with someone who is eccentric or quite old. Can be used in both a positive and negative context.
He’s 80 years old and has lived on his own for 20 years, so it’s hardly surprising that he has an idiosyncrasy or two.
Marie is brilliant at what she does, so her staff don’t mind her minor idiosyncrasies.
The author’s writing style is highly idiosyncratic, as in it’s distinctive to him alone.