precursor precursor /priːˈkɝːsɝ/

noun

1. Something that precedes and indicates the approach of something or someone  ( harbinger , herald , precursor , forerunner , predecessor )
   something that precedes and indicates the approach of something or someone
   
2. A person who goes before or announces the coming of another  ( precursor , forerunner )
   a person who goes before or announces the coming of another
   
3. (biochemistry) a substance from which another substance is formed (especially by a metabolic reaction)  ( precursor )
   (biochemistry) a substance from which another substance is formed (especially by a metabolic reaction)