Solaris is an "industrial-strength" version of UNIX made by SunMicrosystems, who also make hardware. Sun are losing the low-end to much cheaper linux-on-x86 boxes.
The operating system used to be known as SunOS, but "recent" (in Solaris time anyway) versions have been known as Solaris and had their version number bumped up. I think the difference is that Solaris is supposedly the whole operating environment.
So, Solaris 2.8 is also known as SunOS 5.8, and maybe just called "Solaris 8". Similarly Solaris 2.9 is (or contains) SunOS 5.9
Linux has copied many of the features of the Solaris kernel and user-space, such as the style of dynamic linking (eg dlopen(3), ld.so, the LD_PRELOAD environment variable).
29 pages link to Solaris:
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