Upper airway
The airway that runs from the nose and mouth down to the larynx. You can think of it as a flexible tube whose muscles relax during sleep.
A little more detail
Consists of the nose, nasopharynx, oropharynx and hypopharynx; the pharynx is a segment without bony support that stays open through muscle tone. During sleep, reduced dilator muscle activity combined with anatomical narrowing and obesity increases the tendency to collapse.
Source: Dempsey JA, Veasey SC, Morgan BJ, O'Donnell CP. Pathophysiology of sleep apnea. Physiol Rev. 2010;90(1):47-112. doi:10.1152/physrev.00043.2008