The definition of atelectasis is basically loss of air in alveoli.

Diagnosis of atelectasis requires the following:

1. A density, representing lung devoid of air
2. Signs indicating loss of lung volume

If alveoli are filled with fluid, inflammatory fluid or any thing else it is not considered atelectasis. For instance in pneumonia alveoli are filled with inflammatory exudate and is devoid of air. We call it consolidation and not atelectasis. There will not be loss of lung volume if alveoli are filled with any substance.In atelectasis the alveoli are empty.