The method of solving the implicit scheme of difference equations by means of imaginary boundary condition was proposed.

提出了应用“假想边界”求解数学模型的隐式差分格式的方法。

youdao

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

"Feathers are one of the things that transit the imaginary boundary we put between the human and natural world, " he explained.

WSJ: Of Feathers, Showgirls and Biology