Idiosyncrasy

ˌɪd.i.əˈsɪŋ.krə.si

noun

A mode of behavior or way of thought peculiar to an individual.

Idiosyncrasy comes from the Greek word 'idiosynkrasia' meaning 'a peculiar habit of body' and is commonly used to describe unique characteristics or behaviors of individuals.

See how T. E. Lawrence uses "idiosyncrasy", with its meaning, pronunciation and a real book example.