линия раздела
• There is no sharp line of demarcation (or demarcation line) between the atmosphere and space.
• The two parts are separated by a dividing line.
Русско-английский научно-технический словарь• There is no sharp line of demarcation (or demarcation line) between the atmosphere and space.
• The two parts are separated by a dividing line.
Русско-английский научно-технический словарь