definer

noun

1. One who or that which defines something, or provides a definition.

   “It served as a key definer of national character, which was in Zelter's time emerging out of a combination of folk and elite traditions […]”

   “Leaving aside the abundance of negativity for the time being, in aggregate these attempts at definition speak to the multitude of linguistic phenomena characteristic of language hybridity in multilingual settings, albeit explained with differing emphases by different definers.”