To play "air guitar" you move to the music as if you are playing an imaginary guitar, maybe like a famous rock musician.

VOA: special.2009.08.14

It doesn't have to be a real container to contain it It's just an imaginary boundary there.

它不必是,一个真实的容器,只是一个想象的边界。

麻省理工公开课 - 热力学与动力学课程节选

I heard one of them say to an imaginary suitor, "and you without health or a home."

我听到有人对臆想的原告说，你没有健康，你没有家庭“

耶鲁公开课 - 现代诗歌课程节选

Meanwhile, his youth was going away and now this imaginary student is now thirty-three years old never had time to marry or start a normal life.

与此同时，他的青春年华也已经逝去了,现在这位设想出来的同学已经33岁了,还没有结婚也没有开始正常人的生活。

耶鲁公开课 - 金融市场课程节选

If you had one negative number in it, then the product would be a negative number and, if you took a root of that, then you might get an imaginary number.

如果这当中有一个负数,乘积就会变成负的,如果你对一个负数开根号,你就会得到一个虚数

耶鲁公开课 - 金融市场课程节选