The the company that owned UNIX at the time, AT&T, made it more and more difficult to get this distribution, so various people replaced the AT&T portions of BSD with rewrites from scratch that were often superior. "BSD UNIX" became "UNIX" in name only. When AT&T, then owner of the trademark "UNIX", complained about this, people stopped using the name "UNIX" and called the operating system simply "BSD".
Free versions of BSD are available from these sites:
A commercial version of BSD is available from BSDI
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