noun
a characteristic of temperament, habit, or physical structure particular to a given individual or group; peculiarity.
Growing antlers is an idiosyncrasy of female reindeer; in general, female deer do not grow antlers.
One of the inventor’s idiosyncrasies was that he rarely felt the need to sleep.
Scrunching up her nose was an idiosyncrasy of hers that we often teased her about.