A yawn is a reflex in vertebrate animals characterized by a long inspiratory phase with gradual mouth gaping, followed by a brief climax (or acme) with muscle stretching, and a rapid expiratory phase with muscle relaxation, which typically lasts a few seconds. For fish and birds, this is described as gradual mouth gaping, staying open for at least three seconds and subsequently a rapid closure of the mouth. Almost all vertebrate animals, including mammals, birds, reptiles, amphibians, and even fish, experience yawning. The study of yawning is called chasmology.

Yawning (oscitation) most often occurs in adults immediately before and after sleep, during tedious activities and as a result of its contagious quality. It is commonly associated with tiredness, stress, sleepiness, boredom, or even hunger. In humans, yawning is often triggered by the perception that others are yawning (for example, seeing a person yawning, or talking to someone on the phone who is yawning). This is a typical example of echopraxia and positive feedback. This "contagious" yawning has also been observed in chimpanzees, dogs, cats, birds, and reptiles and can occur between members of different species. Approximately twenty psychological reasons for yawning have been proposed by scholars, but there is little agreement on the primacy of any one.

During a yawn, muscles around the airway are fully stretched, including chewing and swallowing muscles. Due to these strong repositioning muscle movements, the airway (lungs and throat) dilates to three or four times its original size. The tensor tympani muscle in the middle ear contracts, which creates a rumbling noise perceived as coming from within the head; however, the noise is due to mechanical disturbance of the hearing apparatus and is not generated by the motion of air. Yawning is sometimes accompanied, in humans and other animals, by an instinctive act of stretching several parts of the body including the arms, neck, shoulders and back. In humans it is often visible that nostrils are dilating involuntary during yawning.

The English yawn continues a number of Middle English forms: yanen from Old English ġānian, and yenen, yonen from Old English frequentatives ġinian, ġionian, from a Germanic root *gīn-. The Germanic root has Proto-Indo-European cognates, from a root *g̑hēi- found also with -n- suffix in Greek χαίνω ('to yawn'), and without the -n- in English gap (compare the figura etymologica in Norse ginnunga-gap), gum ('palate') and gasp (via Old Norse), Latin hiō, hiatus, and Greek chasm, chaos.

The Latin term used in medicine is oscitatio (anglicized as oscitation), from the verb oscito ('to open the mouth'). Pandiculation is the act of yawning and stretching simultaneously.

There are a number of theories that attempt to explain why humans and other animals yawn.

One study states that yawning occurs when one's blood contains increased amounts of carbon dioxide and therefore becomes in need of the influx of oxygen (or expulsion of carbon dioxide) that a yawn can provide. Yawning may reduce oxygen intake compared to normal respiration; however, the frequency of yawning is not decreased by providing more oxygen or reducing carbon dioxide in the air.

Animals subject to predation or other dangers must be ready to physically exert themselves at any given moment. At least one study suggests that yawning, especially psychological "contagious" yawning, may have developed as a way of keeping a group of animals alert. If an animal is drowsy or bored, it will be less alert than when fully awake and less prepared to spring into action. "Contagious" yawning could be an instinctual signal between group members to stay alert.