An Etymological Dictionary of Astronomy and Astrophysics

1. Something that stands for or represents something else, especially an object representing an abstraction.
   
2. A conventional sign or character that represents something in a specific context, e.g. an operation or quantity in mathematics or music.

Etymology (EN): M.E., from L.L. symbolum “creed, token, mark,” from Gk. symbolon “sign, mark,” from → syn- “together” + stem of ballein “to throw.”

Etymology (PE): Namâd variant of namud, nemud past stem of nemudan “to show;” Mid.Pers. nimūdan, nimây- “to show,” from O.Pers./Av. ni- “down; into,” → ni- (PIE), + māy- “to measure;” cf. Skt. mati “measures,” matra- “measure;”
Gk. metron “measure;” L. metrum; PIE base *me- “to measure.”