noun (n)

• a number or ratio (a value on a scale of measurement) derived from a series of observed facts; can reveal relative changes as a function of time(noun.communication)

  Synonym:
  index, index number, indicant
  

  source: wordnet30
• a signal for attracting attention(noun.communication)

  source: wordnet30
• a device for showing the operating condition of some system(noun.artifact)

  source: wordnet30
• (chemistry) a substance that changes color to indicate the presence of some ion or substance; can be used to indicate the completion of a chemical reaction or (in medicine) to test for a particular reaction(noun.substance)

  source: wordnet30
• One who, or that which, shows or points out; as, a fare indicator in a street car.(noun)

  source: webster1913