System V (System 5) was an early form of the Unix operating system, originally developed by AT&T (American Telephone and Telegraph). The first release, Release 1 (SVR1), appeared in 1983. Release 2 (SVR2) followed in 1984, Release 3 (SVR3) in 1987, and Release 4 (SVR4, the last and most popular version) in 1990. The original System V is no longer produced, but derivatives exist, most notably OpenServer and UnixWare from the SCO Group and Solaris from Sun Microsystems.
System V has been compared to BSD (which originally stood for Berkeley Software Distribution), another "flavor" of Unix. The first three versions of System V were preferred by businesses, while BSD was favored by university professors and research scientists. After the release of SVR3, AT&T began working with Sun Microsystems to produce a version of System V that would appeal to a wider group of users. The result was SVR4, which included many of the features of BSD, as well as characteristics of the Sun operating system as it existed at that time. In 1993, AT&T sold its interest in SVR4 Unix to Novell, who later sold their interest to the SCO Group.