southwestern United States vs Southwest

southwestern United States and Southwest both are nouns.

Word	Noun	Adjective	Verb	Adverb
southwestern United States	Yes	No	No	No
Southwest	Yes	No	No	No

As nouns, southwestern United States and Southwest are synonyms defined as:

southwestern United States and Southwest: the southwestern region of the United States generally including New Mexico, Arizona, Texas, California, and sometimes Utah and Colorado

southwestern United States (noun)	Southwest (noun)
the southwestern region of the United States generally including New Mexico, Arizona, Texas, California, and sometimes Utah and Colorado	the southwestern region of the United States generally including New Mexico, Arizona, Texas, California, and sometimes Utah and Colorado