This article is not about the fictional alien race. See Precursor (Star Control). for that information

General

In generic terms, a precursor is something that existed before and was incorporated into something that came later. Also known as a forerunner or predecessor .

As an example, cacao is a precursor of chocolate. But chocolate is not a precursor of ice cream. There, chocolate is an ingredient.

Biology

In biological processes especially metabolism, a precursor is a substance from which another, usually more active or mature substance is formed.

For instance, certain liver enzymes are precursors to insulin. Or, this is the opposite of the body breaking down a more complex substance, like protein into individual chemicals for use within the body.

Chemistry

Chemically, a precursor is a substance that, following a reaction, becomes an intrinsic part of a product chemical.

This is often heard when discussing methamphetamine production, as certain household items are precursors of the end product.

Medicine

Medically, symptoms that, following the a certain progression, indicate the development of a different problem are also called 'precursors.

For example, chest pain can be a precursor to a heart attack. A heart attack involves chest pains, but not all chest pains indicate a heart attack.