DOSBox is a free and open-source emulator program that emulates an IBM PC compatible computer running a DOS operating system. It is designed to run classic DOS-based games and programs on modern computers, while also providing a range of other features including DOS command line emulation, expanded memory support, mouse and joystick support, and the ability to mount virtual hard drives and floppy disk images. DOSBox can run on a variety of operating systems, including Windows, Linux, macOS, and even Android.
DOSBox is an 8086 emulator, not an operating system.