The design of the X Protocol specifies a client-server relationship between an application and its display.

协议的设计把应用程序和它的显示定义为客户机-服务器关系。

youdao

Canonical originally planned to replace the aging X with another display server called Wayland, but the developers apparently couldn't bend the compositing-friendly protocol to their cross-device whims.

ENGADGET: Canonical announces Mir, a custom display server that will serve up future versions of Unity