The term "border line" refers to the point where one thing ends and another begins. There are several synonyms that can be used to express this concept. One possible term is "boundary," which refers to a line that separates two areas or territories. Another option is "frontier," which suggests an area that is on the edge of civilization or exploration. "Margin" is also a synonym for "border line," indicating the space around the edge of something. Finally, "threshold" can be a useful synonym, particularly when discussing a point at which a change is about to occur. By using these synonyms, one can add variety and nuance to their writing.