noun
boundary line; border line
Refers to a physical or conceptual dividing line, such as between properties, regions, or abstract categories.
この川が二つの県の境界線になっている。
This river serves as the boundary line between the two prefectures.
境界線を越えて隣の土地に入ら
Please do not cross the boundary line onto the neighboring property.
国境 refers specifically to a national border, whereas 境界線 can be used for any kind of boundary line.
Compound of 境界 (boundary) + 線 (line). The exact historical derivation is straightforward; the spelling is conventionally associated with the meaning.