The boundary is impervious to transfer of heat like a thermos Anything that happens inside of the thermos is an adiabatic change because the thermos has no connection in terms of energy to the outside world.

边界不能传递热量,像热水瓶一样,热水瓶里发生的任何事,都是绝热变化,因为热水瓶与外部世界,没有能量方面的联系。

麻省理工公开课 - 热力学与动力学课程节选