An active ingredient (AI), is the substance in a pharmaceutical drug or a pesticide that is biologically active. Terms in similar use include: active pharmaceutical ingredient (API) and bulk active in medicine, or in pesticide formulations active substance may be used. Some medications and pesticide products may contain more than one active ingredient. The traditional word for the API is pharmacon or pharmakon (from Greek: (φάρμακον), adapted from pharmacos) which originally denoted a magical substance or drug.
A dosage form of a drug is traditionally composed of two things: The API, which is the drug itself; and an excipient, which is the substance of the tablet, or the liquid the API is suspended in, or other material that is pharmaceutically inert. Drugs are chosen primarily for their active ingredients.