An involuntary intake of breath through a wide open mouth; usually triggered by fatigue or boredom WordNet 3.0 & Open English WordNet

A yawn is a reflex in vertebrate animals characterized by a long inspiratory phase with gradual mouth gaping, followed by a brief climax with muscle stretching, and a rapid expiratory phase with muscle relaxation, which typically lasts a few seconds. Wikipedia

Reflex Wikidata

The action of yawning; opening the mouth widely and taking a long, rather deep breath, often because one is tired or bored. Wiktionary

The action of yawning. Wiktionary (translation)

The action of the verb yawn. Wiktionary