noun
breathing of a sleeping person; sleeping breath
Refers to the soft, steady sound or rhythm of someone's breathing while asleep. Often used in quiet, peaceful descriptions.
赤ちゃんの寝息が聞こえてきた。
I could hear the baby's sleeping breath.
彼の寝息はとても静かだ。
His breathing while asleep is very quiet.
いびき refers specifically to snoring, which is loud and often disruptive, while 寝息 is the gentle, quiet breathing of someone sleeping peacefully.
Compound of 寝 (ne, 'sleep') and 息 (iki, 'breath'), with the reading iki becoming iki due to rendaku. The exact historical derivation is straightforward.