The Sinclair QL's proprietary operating system. The origin of the name is uncertain (a weak pun on kudos, perhaps, as Unix was on Multics). There was another OS around from the birth of personal computers called Q.D.O.S. - Quick And Dirty Operating System. QDOS might also stand for QL Data/Disk/Drive/Device Operating System.
QDOS did the usual OS sorts of things, as well as multitasking. It was unusual in several ways. It treated all devices (serial ports, mouse ports, screen, microdrive, disk drive, keyboard, etc.) uniformly, so you could print a text file direct to disk or save a binary to the screen for example.
Also logical channels could be assigned to particular physical devices. Output directed to a channel would go to the appropriate in/output. This also meant you could have many windows on screen (the QL booted up from internal ROMs with 3 windows - command line, output and program listing) all independent to some extent. Channels could be redirected without affecting the way the process sent or received the data.