frontera
/fɾonˈteɾa/ · B1 · noun · f
boundary line
🚧 frontera = front + -era: the front edge of a country, like English frontier
From Latin frons, frontis (forehead, front) via Spanish frente. English frontier is the same word through French. The border is where one territory fronts onto the next; swap -ier for -era and you have it.