An idiosyncrasy is an unusual feature of a person (though there are also other uses, see below). It can also mean an odd habit. The term is often used to express eccentricity or peculiarity. A synonym may be "quirk". Etymology The term "idiosyncrasy" originates from Greek , "a peculiar temperament, habit of body" (from , "one's own", , "with" and , "blend of the four humors" (temperament)) or literally "particular mingling". Linguistics 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. Idiosyncratic property In phonology, an idiosyncratic property contrasts with a systematic regularity. While systematic regularities in the sound system of a language are useful for identifying phonological rules during analysis of the forms morphemes can take, idiosyncratic properties are those whose occurrence is not determined by those rules. For example, the fact that the English word cab starts with a /k/ is an idiosyncratic property; on the other hand that its vowel is longer than in the English word cap is a systematic regularity, as it arises from the fact that the final consonant is voiced rather than voiceless. Idiosyncrasy in religion Among religious people, idiosyncrasy means behavior that is different from the usual, different from that which is generally seen as common, yet also as different social behavior that can be seen in the varieties of cults. Medicine Disease 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. Pharmacology The term idiosyncratic drug reaction denotes an aberrant or bizarre reaction or hypersensitivity to a substance, without connection to the pharmacology of the drug. It is what is known as a Type B reaction. Type B reactions have the following characteristics: they are usually unpredictable, might not be picked up by toxicological screening, not necessarily dose-related, incidence and morbidity low but mortality is high. Type B reactions are most commonly immunological (e.g. penicillin allergy). Psychiatry and psychology In psychiatry, the term means a specific and unique mental condition of a patient, often accompanied by neologisms.