What is an active pharmaceutical ingredient (API)?

Definition of API in the EU GMP Guide Part II: Any substance or mixture of substances intended to be used in the manufacture of a drug (medicinal) product/ finished pharmaceutical product (FPP) and that, when used in the production of a drug, becomes an active ingredient of the drug product. Such substances are intended to furnish pharmacological activity or other direct effect in the diagnosis, cure, mitigation, treatment, or prevention of disease or to affect the structure and function of the body.

[GMP Compliance Adviser, Chapter 20.A]