SunMicrosystems' (current) interpretation of the UnixOperatingSystem for its hardware (and sometimes others' as well).
Solaris currently runs both on x86 (including x86-64) and Sun's own Sparc CPUs. Sun has blown hot and cold on x86 Solaris over the years; as of 2006 it's keenly (and quite successfully) promoting its own x86-64 hardware running Solaris 10.
Sun has had two Unices. Up to version 4.0 SunOs was a BSD variant. SunOs 5.0 (released in 1992) was a different beast, based on SystemFive?. (SunOs 4.0 was retroactively christened Solaris v. 1 nonetheless.) All the subsequent versions (up to 2006) are descendants of 5.0.
Since June 2005, most of Solaris is now open source, under Sun's somewhat-controversial CDDL license - see http://www.opensolaris.org/ . This has contributed to a bit of a revival in Solaris' fortunes, though (as of 2006) Linux is still much more ubiquitous.