a. The action or process of inhaling and exhaling; breathing. Also called ventilation.
b. An act of inhaling and exhaling; a breath.
2. The action or process by which an organism without lungs, such as a fish or plant, exchanges gases with its environment.
a. 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.
b. 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.
The American Heritage® Dictionary of the English Language, Fifth Edition copyright ©2020 by Houghton Mifflin Harcourt Publishing Company. All rights reserved.
Indo-European & Semitic Roots Appendices
Thousands of entries in the dictionary include etymologies that trace their origins back to reconstructed proto-languages. You can obtain more information about these forms in our online appendices:
The Indo-European appendix covers nearly half of the Indo-European roots that have left their mark on English words. A more complete treatment of Indo-European roots and the English words derived from them is available in our Dictionary of Indo-European Roots.