In De 1:1; 2:8, "plain" is translated in the Revised Version (British and American) "Arabah," and explained, "the deep valley running North and South of the Dead Sea." It was here that Moses delivered his last addresses. Ususally the word is plural (`arebhoth), the "plains" or steppes of Moab (Nu 22:1, etc.; De 34:1,8). An interesting description is given in an article on "The Steppes of Moab" by Professor G. B. Gray in The Expositor, January, 1905.
See MOAB.