To open the mouth widely and take a long, rather deep breath, often because one is tired or bored, and sometimes accompanied by pandiculation.

“I could see my students yawning, so I knew the lesson was boring them.”

To say while yawning.

To present a wide opening.

“Death yawned before us, and I hit the brakes.”

The action of the verb yawn.

That yawns or yawn.

“yawning commuters”

Wide open.

“a yawning chasm;  the shark's yawning jaws”