Something that precedes. In biological processes, a substance from which another, usually more active or mature substance is formed. In clinical medicine, a sign or symptom that heralds another.
In chemistry, a substance transformed into another substance by chemical reaction. The substance x is a compound that is transformed into substance x by chemical reaction.
A substance or cell from which another substance or cell is developed, e.g. dopa, the precursor for dopamine, which is converted to dopamine by the enzyme dopa decarboxylase.
A compound which can be used by the body to make another compound or nutrient, such as a vitamin. An example is B-carotene, a precursor of vitamin A; another is ergosterol, present in the skin, which is converted into vitamin D on exposure to sunlight.
A substance that precedes another substance (e.g., in a series of chemical reactions); or a substance from which another is synthesized.
A nutrient or biochemical required for the manufacture of a particular substance within the body (e.g. carrots contain the precursor substance carotene, which is converted in the body to vitamin A).