a line; a boundary; a dividing line
Often used figuratively for a clear boundary or distinction, such as between two concepts, groups, or stages. Common in phrases like 一線を画す (draw a line, make a distinction).
プロとアマの間には明確な一線がある。
There is a clear line between professionals and amateurs.
His work stands apart from that of other writers.
His work stands clearly apart from other artists' work.
The boundary between work and private life is becoming blurred.
cross the line; go too far