The Collaborative International Dictionary of English v.0.48:

Southwestern \South`west"ern\, a.
   Of or pertaining to the southwest; southwesterly; as, to sail
   a southwestern course.
   [1913 Webster] Southwestward

WordNet (r) 3.0 (2006):

southwestern
    adj 1: situated in or oriented toward the southwest [syn:
           southwest, southwestern, southwesterly]
    2: of a region of the United States generally including New
       Mexico; Arizona; Texas; California; and sometimes Nevada;
       Utah; Colorado
    n 1: a dialect of Middle English [syn: Southwestern, West
         Saxon]