Definition
A multiuser system is an operating system designed from the ground up to support multiple simultaneous human users, each with a distinct identity, private files, and an isolated session. The OS knows who owns each process, who owns each file, and what permissions each user has — and it enforces those boundaries even when users share the same hardware at the same time.
Unix was multiuser from its first release in 1969, designed for shared mainframes and timesharing servers where dozens of users might be logged in concurrently through terminals. That heritage shapes Linux today. Even on a single-person laptop, every process runs as some user, every file has an owner and a permission mask, and the kernel enforces those rules constantly.
Why it matters
How it works
Every process the OS runs has an associated user ID (uid) and group ID (gid). Every file in the filesystem has an owning uid and gid stored in its inode, plus a 9-bit permission mask that specifies what the owner, the owning group, and everyone else can do. When a process tries to read, write, or execute a file, the kernel checks the process's uid against the file's permission mask and either allows or denies the operation. This is the entire access-control story for traditional Unix — small, fast, and conceptually clean.
Special accounts give the model its texture. The root account (uid 0) bypasses permission checks entirely; it can read or modify anything on the system. Service accounts — www-data, postgres, sshd — exist so that long-running daemons can run without root privileges, limiting the damage if they are exploited. The sudo command lets specific users execute specific commands as root after authenticating, which keeps day-to-day work in an unprivileged account while still allowing administrative work. The whole model assumes that the user identity is meaningful, which is why even single-user laptops still create a separate root account behind the scenes.