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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 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.
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.
In contemporary medicine (as of 2007), the term idiosyncratic drug reaction denotes a non-immunological hypersensitivity to a substance, without connection to pharmacological toxicity. 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.
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 called idiosyncratic risk. This risk can be virtually eliminated from a portfolio through diversification—holding multiple securities means the movements of individual securities "cancel out". It is also often called unsystematic or specific risk. In complete markets, there is no compensation for idiosyncratic risk—that is, a security's idiosyncratic risk does not matter for its price. For instance, in a complete market in which the capital asset pricing model holds, the price of a security is determined by the amount of systematic risk in its returns. Net income received, or losses suffered, by a landlord from renting of one or two properties is subject to idiosyncratic risk due to the numerous things that can happen to real property and variable behavior of tenants.