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Idiosyncrasy comes from Greek ιδιοσυνκρασία "a peculiar temperament", "habit of body" (idios "one's own" and sun-krasis "mixture"). It is defined as a structural or behavioral characteristic peculiar to an individual or group. The term can also be applied to symbols. Idiosyncratic symbols mean one thing for a particular person, as a blade could mean war, but to someone else, it could symbolize a knighting. By the same principle, linguists state that words are not only arbitrary, but also largely idiosyncratic signs.

Idiosyncrasy defined the way physicians conceived diseases in the nineteenth 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.

In contemporary medicine (as of 2007), the term Idiosyncratic drug reaction denotes a non-immunological hypersensitivity to a substance, without connection to pharmacological toxicity.^[1]. Idiosyncratic stresses here the fact that other individuals would react differently, or not at all, and that the reaction is an individual one based on a specific condition of the one who suffers it. Most commonly, this is caused by an enzymopathy, congenital or acquired, so that the triggering substance cannot be processed properly in the organism and causes symptoms by accumulating or blocking other substances to be processed. An idiosyncrasy causing symptoms like an allergy is also called pseudoanaphylaxis ^[1].

In psychiatry, the term means a specific and unique mental condition of a patient, often accompanied by neologisms. In psychoanalysis and behaviorism, it is used for the personal way a given individual reacts, perceives and experiences a common situation: a certain dish made of meat may cause nostalgic memories in one person and disgust in another. These reactions are called idiosyncratic.

In portfolio theory, risks of price changes due to the unique circumstances of a specific security, as opposed to the overall market, are described as idiosyncratic risk. This risk can be virtually eliminated from a portfolio through diversification. It is also often called unsystematic or specific risk. It means there is no compensation for risk, no matter how risky the asset is, and no matter how risk averse we are.

In econometrics, idiosyncratic error is used to describe error from panel data that both changes over time and across units (individuals, firms, cities, etc.)

1. ^ ^{a b} Roche Lexikon Medizin, 5th edition (online version, German)