Unix

The original operating system whose usage and functionality Linux, the BSDs, and other modern "unixlike" OSes resemble and have expanded upon. Invented at AT&T's Bell Labs by Dennis Richie and Ken Thompson. It fragmented into BSD (from the SysV release) at Berkley, whereupon it fragmented some more.

For a more complete history of unix, see http://vertigo.hsrl.rutgers.edu/ug/unix_history.html