demarcator

noun

1. One who or that which demarcates.

   “The event [the assassination of John F. Kennedy] was a demarcator. It was the last major public event to come to us in black and white. The urban riots and the Vietnam war, coming right along, were brought to us in living color.”

   “Part of the difficulty with this task, as we have seen, is that there may be no ready demarcators of gender divisions, or, if there are such demarcators, they may not be made along lines that are susceptible of analysis in Eurocentric terms.”