Snoring (or stertor, from Latin stertere 'to snore') is an abnormal breath sound caused by partially obstructed, turbulent airflow and vibration of tissues in the upper respiratory tract (e.g., uvula, soft palate, base of tongue) which occurs during sleep. It usually happens during inhalations (breathing in).

Primary snoring is snoring without any associated sleep disorders and usually without any serious health effects. It is usually defined as apnea–hypopnea index score or respiratory disturbance index score less than 5 events per hour (as diagnosed with polysomnography or home sleep apnea test) and lack of daytime sleepiness.

Snoring may also be a symptom of upper airway resistance syndrome or obstructive sleep apnea (apneic snoring). In obstructive sleep apnea, snoring occurs in combination with breath holding, gasping, or choking.

In the International Classification of Sleep Disorders third edition (ICSD-3), snoring is listed under "Isolated symptoms and normal variants" in the section "Sleep-related breathing disorders". The manual defines snoring as "a respiratory sound generated in the upper airway during sleep that typically occurs during inspiration but may also occur in expiration."

Primary snoring (also termed simple snoring, non-apneic snoring, or isolated snoring) is snoring without any other associated medical condition. Primary snoring is not associated with episodes of sleep apnea (cessation of breathing), hypopnea, respiratory-effort related arousals, or hypoventilation. Traditionally, primary snoring is considered as benign and having no significant health effects for the individual. However, the idea that primary snoring without sleep apnea has no negative health effects is being increasingly challenged, especially primary snoring in children. For example, there is evidence that primary snoring causes excessive daytime sleepiness, and it may be linked with several other medical problems, some of which are serious. Even so, it is generally accepted that primary snoring cannot be diagnosed in the presence of sleep apnea.

Snoring is one of the main symptoms of obstructive sleep apnea, in which case it is apneic snoring. In obstructive sleep apnea, snoring occurs in combination with other features such as breath holding (breathing cessation), gasping, or choking. There are also other features like daytime sleepiness, non-restorative sleep, fatigue, or insomnia.

Snoring has also been classified according to frequency as occasional snoring (occurring on three nights or less per week) and habitual snoring (occurring on most nights; synonymous with primary snoring).

Snoring has been classified according to apnea–hypopnea index score and severity of associated sleep disorders. Therefore, snoring as a symptom exists as a spectrum of severity, with primary snoring being the least severe, snoring with upper airway resistance syndrome being of intermediate severity, and snoring associated with obstructive sleep apnea being the most medically significant. This spectrum of conditions represents increasing degrees of airway obstruction and severity and frequency of disruption of breathing during sleep.