respiration

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

• noun The action or process of inhaling and exhaling; breathing.
• noun An act of inhaling and exhaling; a breath.
• noun The action or process by which an organism without lungs, such as a fish or plant, exchanges gases with its environment.
• noun The oxidative process occurring within living cells by which the chemical energy of organic molecules is converted in a series of metabolic steps into usable energy in the form of ATP, involving the consumption of oxygen and the production of carbon dioxide and water as byproducts.
• noun Any of various analogous metabolic processes by which certain organisms, such as anaerobic bacteria and some fungi, obtain energy from organic molecules without consuming oxygen.

from The Century Dictionary.

• noun The act of breathing again or resuming life.
• noun The inspiration and expiration of air.
• noun That function by which there takes place an absorption of oxygen from the surrounding medium into the blood with a corresponding excretion of carbon dioxid.
• noun In physiological bot., a process consisting in the absorption by plants of oxygen from the air, the oxidation of assimilated products, and the release of carbon dioxid and watery vapor.
• noun The respiratory murmur.
• noun A breathing-spell; an interval.

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

• noun The act of respiring or breathing again, or catching one's breath.
• noun obsolete Relief from toil or suffering: rest.
• noun obsolete Interval; intermission.
• noun (Physiol.) The act of resping or breathing; the act of taking in and giving out air; the aggregate of those processes bu which oxygen is introduced into the system, and carbon dioxide, or carbonic acid, removed.

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

• noun The process of inhaling and exhaling; breathing, breath.
• noun An act of breathing; a breath.
• noun Any similar process, in organisms that lack lungs, that exchanges gases with its environment.
• noun The process by which cells obtain chemical energy by the consumption of oxygen and the release of carbon dioxide.

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

• noun the metabolic processes whereby certain organisms obtain energy from organic molecules; processes that take place in the cells and tissues during which energy is released and carbon dioxide is produced and absorbed by the blood to be transported to the lungs
• noun the bodily process of inhalation and exhalation; the process of taking in oxygen from inhaled air and releasing carbon dioxide by exhalation
• noun a single complete act of breathing in and out