A boundary line is a demarcation between two areas or properties. There are several synonyms for the word boundary line which can be used interchangeably. One of the most common synonyms is "border" which is used to describe the edge or limit of something. Another synonym is "frontier" which refers to the unexplored or undiscovered borders of a region. The term "limit" is often used to indicate the extent or range of a particular area. Additionally, "perimeter" refers to the outer boundary of a given space or property. Finally, "borderline" is often used to describe a situation that is on the edge or verge of going beyond the boundaries.