1. 1. The replacement of stale or noxious air with fresh air.
   2. The mechanical system or equipment used to circulate air or to replace stale air with fresh air.
2. See respiration (sense ).
3. Aeration or oxygenation, as of blood.

The American Heritage® Dictionary of the English Language, Fourth Edition Copyright © 2007, 2000 by Houghton Mifflin Company. Updated in 2009. Published by Houghton Mifflin Company. All rights reserved.