Dictionary.net

Any system offering the user the choice oftwo operation systems (OSes) under which to start acomputer. A dual boot system allows the user to run programsfor both operating systems on a single computer (though notsimultaneously). The term "multiple boot" or "multiboot"extends the idea to more than two OSes.The OSes are generally unaware of each other's existence.They are installed on separate hard disk partitions or onseparate disks. They may be able to access each other'sfiles, possibly via some extra driver software if they usedifferent file systems.The OSes need not be completely different - they might bedifferent versions of Microsoft Windows (e.g. Windows XPand Windows NT) or Linux (e.g. Debian and Fedora).A dual boot system differs from an emulator such asvmware, which runs one or more OSes "on top" of the primaryOS, using its resources.