A Yawn is a largely involuntary (although controllable) response where an individual opens his mouth wide and breaths in air, followed by exhaling. A certain amount of noise is often associated with the latter part.
Yawning has been linked to being tired, a lack of oxygen, and boredom. It has also been noted that people have a tendency to yawn if they see others yawning.
A recent theory suggests that yawning is an effort to cool down the brain.