FreeBSD

FreeBSD is a free Unix-like operating system descended from AT&T UNIX via BSD UNIX. Although for legal reasons FreeBSD cannot be called “UNIX,” as the direct descendant of BSD UNIX (many of whose original developers became FreeBSD developers), FreeBSD’s internals and system APIs are UNIX-compliant, and thanks to its permissive licensing terms, much of FreeBSD’s code base has become an integral part of other operating systems such as Mac OS X that have subsequently been certified as UNIX-compliant and have formally received UNIX branding. With the exception of the proprietary Mac OS X, FreeBSD is the most widely used BSD-derived operating system in terms of number of installed computers, and is the most widely used freely licensed, open-source BSD distribution, accounting for more than three quarters of all installed systems running free, open-source BSD derivatives.