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Glossary
For X11 a Screen is an imaginary space where windows can be made placed (mapped) at a range of 2D coordinates. On a physical Xserver (that directly controls output devices), several outputs can be attached to the screen, for showing the picture on actual monitors (see Xrandr).
Each Xserver may support multiple screens, individual windows can be mapped onto any screen (within the same Xserver), but at one at most. Windows also can be positioned at any 2D coordinate on the screen space (even invisible positions).
On a physical Xserver, several outputs can be attached to a Screen. Those usually correspond to actual connectors/monitors. Each output is presenting a rectangular region of the Screen space - these can be positioned arbitrarily (even overlapping, eg. mirroring) and using different resolutions. The actual configuration can be changed at runtime via Xrandr protocol (within the constraints of the actual hardware)
An X11 protocol extension for retrieving and changing output configuration within an Xserver. For each output, it allows changing position, resolution, refresh frequency and transformations (eg. rotation), zoom factor, etc (within the given physical constraints of the actual hardware).
Each window be either "unmapped" or "mapped" onto a particular X11 screen. Unmapped windows are like floating in outer space, not visible and not receiving input, but despite that, still fully functional (eg. holding properties, window-to-window communication, etc). Windows can be mapped onto any of the available screens (within the same Xserver) and thus made be visible.
In a multi-screen configuration (rare today, in contrast to multi-output on one screen), window managers can use (un)mapping to move a window from one screen to another.
An X11 protocol extension for mapping multiple screens into one. It works by replicating all windows on all secondary screens and tweaking their positions so that the user has the impression of all screens being just one.
Today, only rarely used, because most GPUs have multiple outputs, so multi-head setups (multiple physical monitors) can be supported by one X11 screen.