verb

• (intransitive) To open the mouth wide, especially involuntarily, as in a yawn, anger, or surprise.
  "1723, Jonathan Swift, The Journal of a Modern Lady, 1810, Samuel Johnson, The Works of the English Poets, from Chaucer to Cowper, Volume 11, page 467, She stretches, gapes, unglues her eyes, / And asks if it be time to rise;"
• (intransitive) To stare in wonder.
  "Home I vvould go, / But that my Dores are hatefull to my eyes. / Fill'd and damm'd up vvith gaping Creditors, / VVatchfull as Fovvlers vvhen their Game vvill ſpring; […]"
• (intransitive) To open wide; to display a gap.
  "The wound was gaping open and losing too much blood."
• (intransitive, of a cat) To open the passage to the vomeronasal organ, analogous to the flehming in other animals.

noun

• (uncommon) An act of gaping; a yawn.
  "Now a gen'ral gape goes round, And vapours cloud each sleepy head."
• (uncountable) A disease in poultry caused by gapeworm in the windpipe, a symptom of which is frequent gaping.
• (zoology) The maximum opening of the mouth (of a bird, fish, etc.) when it is open.