borderline; boundary
二国の境目に川が流れている。
A river runs along the border between the two countries.
昼と夜の境目は薄明かりの時間だ。
The boundary between day and night is the time of twilight.
仕事とプライベートの境目をなくすのは良くないと思う。
I think it's not good to erase the boundary between work and personal life.