Source : Webster's Revised Unabridged Dictionary (1913)

Precursor \Pre*cur"sor\, n. [L. praecursor, fr. praecurrere to
   run before; prae before + currere to run. See {Course}.]
   One who, or that which, precedes an event, and indicates its
   approach; a forerunner; a harbinger.

         Evil thoughts are the invisible, airy precursors of all
         the storms and tempests of the soul.     --Buckminster.

   Syn: Predecessor; forerunner; harbinger; messenger; omen;
        sign.

Source : WordNet®

precursor
     n 1: a substance from which another substance is formed
          (especially by a metabolic reaction)
     2: a person who goes before or announces the coming of another
        [syn: {forerunner}]
     3: an indication of the approach of something or someone [syn:
        {harbinger}, {forerunner}, {herald}]