An Etymological Dictionary of Astronomy and Astrophysics

1. An object used to indicate a position, place, or route.
   

   2. A distinctive feature or characteristic indicative of a particular quality or condition.
      

   3. Genetics: An allele used to identify a chromosome or to locate other genes on a genetic map (OxfordDictionaries.com).
      

   4. → biomarker.

See also: → mark; → -er.