What is the difference?between atelectasis and collapsed lung?

Grades of the same: Atelectasis (from the latin for imperfect extension) can refer to collapse of a small portion of the lung (called a segment or subsegment) or to collapse of an entire lung. It can be due to obstruction of an airway. Depending on the level of the airway (whether a main bronchus or a smaller bronchiole) depends on the amount of lung that collapses.