Solaris is a Unix operating system originally developed by Sun Microsystems (currently owned by Oracle) as a rebranding of SunOS 4.1.x in 1991.
According to Wikipedia. Solaris was originally developed as a collaboration of Sun Microsystems with major UNIX variants to develop a common platform unifying BSD, System V, and Xenix that has become known as System V Release 4. The first release of Solaris was SunOS 4.1.x which was retroactively renamed Solaris 1. The variant we commonly know as Solaris is actually Solaris 2, which started as SunOS 5 and until version 7 was known as Solaris 2.x (since version 2.7 the prefix 2 has been dropped).
Solaris is a highly scalable Operating System well suited for symmetric multiprocessing systems, which currently includes native virtualization tools called zones.
A Solaris variant called OpenSolaris was released by Sun in 2005, which eventually gave birth to the OpenIndana Project
