In the human respiratory system, eupnea or eupnoea (Greek eupnoia; from eu, well + pnoia, breath) is normal, good, unlabored ventilation, sometimes known as quiet breathing or resting respiration. In eupnea, expiration employs only the elastic recoil of the lungs.
During eupnea, neural output to respiratory muscles is highly regular, with rhythmic bursts of activity during inspiration only to the diaphragm and external intercostal muscles.
Lippincott Williams & Wilkins (2006). Stedman's Medical Dictionary, 28th Edition. Baltimore, Maryland: Julie K. Stegman. pp. 677. ISBN 0-7817-3390-1.