Precursor
Part of speech: noun
Definition: That which precurses, a forerunner, a predecessor, an indicator of approaching events. One of the compounds that participates in the chemical reaction that produces another compound.
Forerunner
Part of speech: noun
Definition: a runner at the front or aheada precursor or harbinger, a warning aheada forebear, an ancestor, a predecessora postage stamp used in the time before a region or area issues stamps of its own