Precursor: Forerunner. That which precedes or is derived from an available source.
The term "precursor" is applied to an inactive substance converted to an active one (such as an enzyme, vitamin, or hormone). The term "precursor" applies to any chemical that is transformed into another.
From the Latin "praecursor," composed of "prae-" (or pre-), before + "curro" to run = to run before.