Jordan takes its name from the Jordan River which forms much of the country's northwestern border.  While several theories for the origin of the river's name have been proposed, it is most plausible that it derives from the Semitic word ''Yarad'', meaning "the descender", reflecting the river's declivity. Much of the area that makes up modern Jordan was historically called ''Transjordan'', meaning "across the Jordan", used to denote the lands east of the river. The Old Testament refers to the area as "the other side of the Jordan". Early Arab chronicles referred to the river as ''Al-Urdunn'', corresponding to the Semitic ''Yarden''. ''Jund Al-Urdunn'' was a military district around the river in the early Islamic era. Later, during the Crusades in the beginning of the second millennium, a lordship was established in the area under the name of ''Oultrejordain''.