'atelectasis' originates from Greek, specifically from the elements 'a-' (not) + 'telos' (complete) combined with 'tasis' (a stretching or expansion), used in medical Greek/Latin formation.

'atelectasis' entered English via modern medical Latin/Greek (used in 19th-century medical literature) formed from Greek elements and was adopted unchanged into English medical vocabulary.

Initially it referred generally to 'incomplete expansion' (of a body part); over time it came to be used specifically for the collapse or failure of expansion of lung tissue.