Idiosyncrasy

Idiosyncrasy is a noun that describes a characteristic peculiarity of an individual or group, especially a trait that is unusual or distinctive. It can refer to a quirky behavior, taste, or habit, and is often used to emphasize that such features are idiosyncratic rather than universal. The term comes from the Greek idios, meaning “one’s own,” combined with krasis (or crasis), meaning “mixture,” implying a blending that is specific to a person or thing.

In medicine, the word has a more technical meaning: an idiosyncratic reaction is an unusual and unpredictable response to a drug or treatment that cannot be explained by the drug’s known pharmacology and is not dose-related. Such reactions are thought to reflect individual genetic, metabolic, or immune differences, and they can complicate therapy.

In everyday and scholarly use, idiosyncrasy also denotes distinctive traits that define a person or culture—habits, preferences, or ways of thinking that set someone apart but are not necessarily negative. Writers sometimes discuss group idiosyncrasies to highlight cultural specificity or individuality.

Related terms include quirk, peculiarity, eccentricity, and anomaly.