lung

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun Either of two spongy, saclike respiratory organs in air-breathing vertebrates, occupying the chest cavity together with the heart and functioning to provide oxygen to the blood while removing carbon dioxide.
• noun A similar organ in some invertebrates, including spiders and terrestrial snails.
• idiom (at the top of (one's) lungs) As loudly as one's voice will allow.

from The Century Dictionary.

• noun One of the two spongy or saccular organs, occupying the thorax or upper part of the body-cavity, which communicate with the pharynx through the trachea, and are the organs of respiration in air-breathing vertebrates.
• noun In entomology, one of the respiratory organs peculiar to those Arachnida whose tracheal system is modified into a number of lamellæ superimposed upon one another like the leaves of a book. They are also called pulmonary lamellæ and respiratory leaflets.
• noun In pulmonate mollusks, a modification of the integument subserving aërial respiration: more fully called external lung. Huxley.
• noun plural A bellows-blower; a chemist's servant.

from the GNU version of the Collaborative International Dictionary of English.

• noun (Anat.) An organ for aërial respiration; -- commonly in the plural.
• noun (Med.) pneumonia.
• noun (Bot.) a species of gentian (Gentian Pneumonanthe).
• noun (Bot.) tree lungwort. See under Lungwort.
• noun (Zoöl.) one of the breathing organs of spiders and snails.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun anatomy A biological organ that extracts oxygen from the air.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun either of two saclike respiratory organs in the chest of vertebrates; serves to remove carbon dioxide and provide oxygen to the blood