idiosyncrasy

A behavior or way of thinking that is characteristic of a person or a group.

This mode of death had been an idiosyncrasy with his family, for generations past; not often occurring, indeed, but, when it does occur, usually attacking individuals about the Judge’s time of life, and generally in the tension of some mental crisis, or, perhaps, in an access of wrath.

Every village has its idiosyncrasy, its constitution, often its own code of morality.

A peculiar individual reaction to a generally innocuous substance or factor; a risk factor.

[…]I have no antipathy, or rather Idio-ſyncraſie, in dyet, humour, ayre, any thing; […].

There are, however, distinct differences between diatheses and idiosyncrasies; diatheses definitely tend to the development of disease, for example the scrofulous diathesis of our forebears favours the onset of tuberculosis; idiosyncrasies are abnormal reactions but do not necessarily dispose to disease.

A peculiarity that serves to distinguish or identify.

He mastered the idiosyncrasies of English spelling and speech.