noun (n)

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

  Synonym:
  precursor
  

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

  Synonym:
  harbinger, herald, precursor, predecessor
  

  source: wordnet30
• anything that precedes something similar in time(noun.linkdef)

  Synonym:
  antecedent
  

  source: wordnet30
• A messenger sent before to give notice of the approach of others; a harbinger; a sign foreshowing something; a prognostic; as, the forerunner of a fever.(noun)

  source: webster1913