yawn

v. involuntarily open the mouth wide and inhale due to drowsiness or fatigue

n. involuntary deep intake of air from drowsiness or fatigue

A yawn (synonyms chasma, pandiculation, oscitation from the Latin verb oscitare, to open the mouth wide ) is a reflex of deep inhalation and exhalation associated with tiredness, stress, over-work, lack of stimulation, or boredom. Pandiculation is the term for the act of stretching and yawning. Yawning is a powerful non-verbal message with several possible meanings, depending on the circumstances. Another speculated reason for yawning is nervousness and is also claimed to help increase the state of alertness of a person - paratroopers were noted yawning right before their first jump. The exact causes of yawning are still undetermined.
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Yawn v. To open the mouth wide with a deep inhalation, usually involuntarily from drowsiness, fatigue, or boredom. n. The act of yawning. [more]Yawn - Community and Resources

Noun
1. an involuntary intake of breath through a wide open mouth; usually triggered by fatigue or boredom; "he could not suppress a yawn"; "the yawning in the audience told him it was time to stop"; "he apologized for his ostinancy"
(synonym) yawning, oscitance, oscitancy
(hypernym) reflex, instinctive reflex, innate reflex, inborn reflex, unconditioned reflex, physiological reaction
(hyponym) pandiculation
Verb
1. utter a yawn, as from lack of oxygen or when one is tired; "The child yawned during the long performance"
(hypernym) breathe, take a breath, respire, suspire
(derivation) yawning, oscitance, oscitancy
2. be wide open; "the deep gaping canyon"
(synonym) gape, yaw
(hypernym) be

(v. i.)
To open wide; to gape, as if to allow the entrance or exit of anything.
  

(v. i.)
To open the mouth, or to gape, through surprise or bewilderment.
  

(v. i.)
To open the mouth involuntarily through drowsiness, dullness, or fatigue; to gape; to oscitate.
  

(v. i.)
To be eager; to desire to swallow anything; to express desire by yawning; as, to yawn for fat livings.
  

(n.)
The act of opening wide, or of gaping.
  

(n.)
An involuntary act, excited by drowsiness, etc., consisting of a deep and long inspiration following several successive attempts at inspiration, the mouth, fauces, etc., being wide open.
  

(n.)
A chasm, mouth, or passageway.