noun (n)

• a substance from which another substance is formed (especially by a metabolic reaction)(noun.substance)

  source: wordnet30
• a person who goes before or announces the coming of another(noun.person)

  Synonym:
  forerunner
  

  source: wordnet30
• something that precedes and indicates the approach of something or someone(noun.communication)

  Synonym:
  forerunner, harbinger, herald, predecessor
  

  source: wordnet30
• One who, or that which, precedes an event, and indicates its approach; a forerunner; a harbinger.(noun)

  source: webster1913