One of the earliest and longest-lasting operating systems. It has an entry in this dictionary for two reasons. The first is historical: most of the protocols that are used on the Internet, including the Transmission Control Protocol and the Internet Protocol, were initially developed for UNIX since its program code was readily available and because it is an easy operating system to modify. The second reason is that one of its derivatives, BSD UNIX, has become one of the two most common Internet programming platforms. See also Linux.