idiosyncracy

Noun

idiosyncrasy (plural idiosyncrasies)

1. 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.1851 April 9, Nathaniel Hawthorne, The House of the Seven Gables, a Romance, Boston, Mass.: Ticknor, Reed, and Fields, →OCLC

   Synonyms:

   eccentricity, foible, habit, idiocrasy, mannerism, oddity, quirk, vagary

2. (originally) 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; […].1642, [Thomas Browne], “The Second Part”, in Religio Medici, London: […] Andrew Crooke, →OCLC, page 111
3. A peculiarity that serves to distinguish or identify.
   • He mastered the idiosyncrasies of English spelling and speech.