Since you have tagged this with "Lousiana Purchase and the Opening of the West," I assume that you are talking about a time in US history rather than about the line drawn by the British with the Proclamation of 1763.
If that is the case, I believe that the term you are looking for is "frontier." The frontier should not be imagined, though, as a line that ran, like a city limit line or a state line, between white settlements and Indian settlements that were right next to each other. Instead, the frontier should be seen as a general area where the farthest-out of the white settlements ended. The frontier was an area that kept moving farther and farther west as the white settlers pushed out into the Louisiana Purchase.