A peculiarity of physical or mental constitution or temperament; a characteristic belonging to, and distinguishing, an individual; characteristic susceptibility; idiocrasy; eccentricity.

"An Eccentricity of Character or Behavior." From the Greek "idiosynkrasia" of the same meaning. The Greek word developed from "idios" = "private or personal" and "synkrasis," which translates roughly to "admixture" from "syn" = "together" and "krasis" = "mixture." See also "idiotsyncrasy."

a behavioral attribute that is distinctive and peculiar to an individual

specific (and usually unexplained) reaction of an individual to e.g. a chemical exposure to which most other individuals do not react at all. Examples: some people react to their first aspirin with a potentially fatal shock. General allergic reactions do not fall into this category.

In terms of medications: an individual’s unique reaction to a particular drug.

Unexpected sensitivity to medicine or food.