Also failure of pulmonary alveoli to expand at birth

• Collapse of an expanded lung (especially in infants)

Wiktionary Word definitions in Wiktionary
n. (context medicine English) The collapse of a part of or the whole lung caused by inner factors rather than a pneumothorax.

Wikipedia Word definitions in Wikipedia
Atelectasis is the collapse or closure of a lung resulting in reduced or absent gas exchange. It may affect part or all of a lung. It is usually not bilateral. It is a condition where the alveoli are deflated down to little or no volume, as distinct from ...

WordNet Word definitions in WordNet
n. collapse of an expanded lung (especially in infants); also failure of pulmonary alveoli to expand at birth

Douglas Harper's Etymology Dictionary Word definitions in Douglas Harper's Etymology Dictionary
"incomplete expansion of the lungs," 1836, medical Latin, from Greek ateles "imperfect, incomplete," literally "without an end," (from a- , privative prefix, + telos "completion") + ektosis "extention." Related: Atelectatic .