<operating system> A term describing an operating system or application program that can be used by several people concurrently; opposite of single-user. Unix is an example of a multi-user operating system, whereas most (but not all) versions of Microsoft Windows are intended to support only one user at a time.
Before Ethernet networks were commonplace, multi-user systems were accessed from a terminal (e.g. a vt100) connected via a serial line (typically RS-232). This arrangement was eventually superseded by networked personal computers, perhaps sharing files on a file server. With the wide-spread availability of Internet connections, the idea of sharing centralised resources is becoming trendy again with cloud computing and managed applications, though this time it is the overhead of administering the system that is being shared rather than the cost of the hardware.
In gaming, both on PCs and games consoles, the equivalent term is multi-player, though the first multi-player games (e.g. ADVENT) were on multi-user computers.