An idiosyncrasy is an unusual feature of a person (though there are also other uses, see below). It also means odd habit. The term is often used to express eccentricity or peculiarity. A synonym may be quirk.

The term idiosyncrasy originates from Ancient Greek ἰδιοσυγκρασία [idiosynkrasía], "a peculiar temperament", "habit of body" (ἴδιος, [idios] "one's own", σύν, [syn] "with" and κρᾶσις [krasis] "mixture").

The term can also be applied to symbols or words. Idiosyncratic symbols mean one thing for a particular person, as a blade could mean war, but to someone else, it could symbolize a surgery. By the same principle, linguists (such as Ferdinand de Saussure) state that words are not only arbitrary, but also largely idiosyncratic signs.^{[citation needed]}

Idiosyncrasy defined the way physicians conceived diseases in the 19th century. They considered each disease as a unique condition, related to each patient. This understanding began to change in the 1870s, when discoveries made by researchers in Europe permitted the advent of a 'scientific medicine', a precursor to the Evidence-Based Medicine that is the standard of practice today.