From LQWiki
A multi-user operating system is one that recognizes different user accounts and has a security model which allows them access to resources, such as CPU time, memory and hard disk space, and the filesystem itself. Linux is a multi-user operating system (although it has a single-user mode for maintenance purposes). All other current major operating systems like the various BSDs and NT) are also multi-user systems, whereas DOS and Windows, for example, were not.
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