Wiktionary
bright line
n. (context idiomatic English) A clear distinction in the context of a legal or moral judgment.
Usage examples of "bright line".
A bright line began at the extreme edge of the desk and began to creep toward the back limits of Laagi territory.
The sound of voices came into the hall from an inner room, from under the door of which light shone in a bright line.
It took them to a place on the coast, and the children were delighted to see the blue sea shining like a thin bright line in the distance.
The door to the audience chamber was closed, and a bright line of light showed over the top.