Xvfb or X virtual framebuffer

https://en.wikipedia.org/wiki/Xvfb

Xvfb or X virtual framebuffer is a display server implementing the X11 display server protocol. In contrast to other display servers, Xvfb performs all graphical operations in virtual memory without showing any screen output. From the point of view of the client, it acts exactly like any other X display server, serving requests and sending events and errors as appropriate. However, no output is shown. This virtual server does not require the computer it is running on to have any kind of graphics adapter, a screen or any input device. Only a network layer is necessary.

An alternative to Xvfb is Xorg with dummy devices. Xvnc can also create a virtual display.

As of 2016, Xvfb is slated to be replaced by xf86-video-dummy in X.org 7.8.