Open Solaris was based on Solaris, which was originally released by Sun in 1991.Solaris is a version of UNIX System V Release 4 (SVR4), jointly developed by Sun and AT&T to merge features from several existing Unix systems.The stable releases of Open Solaris are based on these Nevada builds." The new distribution was planned to refresh the user experience, and would become the successor to Solaris Express as the basis for future releases of Solaris.
The original plan explicitly stated that illumos would not be a distribution or a fork.
However, after Oracle announced discontinuing Open Solaris, plans were made to fork the final version of the Solaris ON kernel allowing illumos to evolve into a kernel of its own. illumos is lightly led by founder Garrett D'Amore and other community members/developers such as Bryan Cantrill and Adam Leventhal, via a Developers' Council.
It was licensed by Sun from Novell to replace Sun OS.
Sun submitted the CDDL (Common Development and Distribution License) to the OSI, which approved it on January 14, 2005.
The bulk of the Solaris system code was released on June 14, 2005.