Grab (X11)
An X client claiming all pointer or keyboard input for itself, regardless of which window has focus.
The X protocol distinguishes active and passive grabs. An active grab (XGrabPointer, XGrabKeyboard) takes effect immediately: an open menu grabs the pointer so that a click outside it goes to the menu, which closes. A passive grab (XGrabKey, XGrabButton) lies in wait for a particular key or button — which is how window managers catch global shortcuts. XGrabServer goes furthest and freezes every other client.
The model assumes clients trust each other. A program that grabs the keyboard and then hangs locks up the whole desktop, and screen lockers like XScreenSaver can't lock if someone else already holds a grab. That any client could listen in on the keyboard was one of the concrete reasons Wayland was built.