to open your mouth wide and breathe in deeply because you are tired or bored

打呵欠:

Alan stretched and yawned.

艾伦伸了伸懒腰，打了个呵欠。

yawning gap / gulf / chasm (between sth), a very large difference between two groups, things, or people

（两者之间的）巨大差距:

the yawning gap between the two parties

两个党派之间的鸿沟

the yawning gulf between the rich and the poor

巨大的贫富差距

[literary] to be or become wide open, especially in a frightening way

张得很开; 豁开〔尤指令人恐惧的〕:

The pit yawned open in front of them.

那深坑在他们面前张着大口。

the yawning gap between the two cliffs

两个悬崖之间的巨大豁口

[C] an act of yawning

呵欠:

Kay shook her head and stifled a yawn (=tried to stop yawning) .

凯晃晃脑袋，忍住了呵欠。

a yawn, [informal] someone or something that is boring

乏味的人（事物）:

The party was a big yawn.

这次聚会乏味透了。