High Quality Content by WIKIPEDIA articles! Unix System V, commonly abbreviated SysV (and usually pronounced though rarely written as "System 5"), is one of the versions of the Unix operating system. It was originally developed by American Telephone & Telegraph (AT&T) and first released in 1983. Four major versions of System V were released, termed Releases 1, 2, 3 and 4. System V Release 4, or SVR4, was commercially the most successful version, being the result of an effort, marketed as Unix System Unification, which solicited the collaboration of the major Unix vendors. It was the source of several commercial common Unix features. While AT&T sold their own hardware that ran System V (see AT&T Computer Systems), most customers ran a version from a reseller, based on AT&T's reference implementation. A standards document called the System V Interface Definition outlined the default features and behavior of implementations. The most widely used versions of System V today are IBM's AIX, based on System V Release 3, and Sun's OpenSolaris and HP's HP-UX, both based on System V Release 4.