BSD stands for Berkley Software Distribution and it’s a particular version of the UNIX Operating System which was developed by the University of California in Berkley. This distribution is at the basis of several free versions of the OS: FreeBSD, OpenBSD, NetBSD, FreeNAS and many more.
The first version was published in 1977 as a slightly modified version (by means of specific patches) of the UNIX release by AT&T. The system underwent several releases during the ‘80s until the 1995 definitive 4.4-lite Release2 release, from which descendant projects have spurt.