What is Active Ingredient?

Legal Definition
An active ingredient (AI) is the ingredient in a pharmaceutical drug that is biologically active. The similar terms active pharmaceutical ingredient (API) and bulk active are also used in medicine, and the term active substance may be used for natural products. Some medication 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.

The term active constituent is often chosen when referring to the active substance of interest in a plant (such as salicylic acid in willow bark or arecoline in areca nuts), because the word ingredient in many minds connotes a sense of human agency (that is, something that a person combines with other substances), whereas the natural products present in plants were not added by any human agency but rather occurred naturally ("a plant doesn't have ingredients").

In contrast with the active ingredients, the inactive ingredients are usually called excipients in pharmaceutical contexts. The main excipient that serves as a medium for conveying the active ingredient is usually called the vehicle. Petrolatum and mineral oil are common vehicles.
-- Wikipedia
Legal Definition
Chemical components that help an product to perform its function. This is the opposite of inactive ingredients.