Southwest vs southwestern United States

Southwest and southwestern United States both are nouns.

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

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

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

Southwest (noun)	southwestern United States (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