— см. Фронт.
Самойлов К. И.Морской словарь. - М.-Л.: Государственное Военно-морское Издательство НКВМФ Союза ССР,1941
There is no sharp line of demarcation (or demarcation line) between the atmosphere and space.
The two parts are separated by a dividing line.
linea di separazione