at·e·lec·ta·sis (?t?l-?k?t?-s?s)n. pl. at·e·lec·ta·ses (-s?z?) 1. The absence of gas from all or part of the lung, due to failure of expansion of the alveoli.2. A congenital condition characterized by incomplete expansion of the lungs.[New Latin : Greek atel?s, incomplete (a-, not; see a-1 + telos, end; see telo-) + Greek ektasis, stretching out (from ekteinein, to stretch out : ek-, out; see ecto- + teinein, to stretch; see epitasis).]American Heritage® Dictionary of the English Language, Fifth Edition. Copyright © 2016 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.atelectasis (?æt??l?kt?s?s) n1. (Pathology) failure of the lungs to expand fully at birth2. (Pathology) collapse of the lung or a part of the lung, usually caused by bronchial obstruction[C19: New Latin, from Greek atel?s imperfect + ektasis extension]Collins English Dictionary ? Complete and Unabridged, 12th Edition 2014 © HarperCollins Publishers 1991, 1994, 1998, 2000, 2003, 2006, 2007, 2009, 2011, 2014at?e?lec?ta?sis (?æt l??k t? s?s) n. 1. incomplete expansion of the lungs at birth, as from lack of breathing force. 2. collapse of the lungs, as from bronchial obstruction. [1855?60;