Final answer:
An idiosyncrasy is an unusual or unexpected drug reaction by an individual that is not related to the drug's known pharmacological effects.
Step-by-step explanation:
The unusual or unexpected drug reaction by an individual is known as idiosyncrasy. It is a unique and unpredictable response to a drug that is not related to the drug's known pharmacological effects. An idiosyncratic drug reaction can vary from person to person and may have different manifestations, such as severe allergic reactions or toxic effects.