Atelectasis is the collapse or closure of a lung resulting in reduced or absent gas exchange. It is usually unilateral, affecting part or all of one lung. It is a condition where the alveoli are deflated down to little or no volume, as distinct from pulmonary consolidation, in which they are filled with liquid. It is often called a collapsed lung, although that term may also refer to pneumothorax.

It is a very common finding in chest X-rays and other radiological studies, and may be caused by normal exhalation or by various medical conditions. Although frequently described as a collapse of lung tissue, atelectasis is not synonymous with a pneumothorax, which is a more specific condition that can cause atelectasis. Acute atelectasis may occur as a post-operative complication or as a result of surfactant deficiency. In premature babies, this leads to infant respiratory distress syndrome.

The term uses combining forms of atel- + ectasis, from Greek: ἀτελής, "incomplete" + ἔκτασις, "extension".