southwestern United States vs California

southwestern United States and California both are nouns.

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

southwestern United States (noun)	California (noun)
the southwestern region of the United States generally including New Mexico, Arizona, Texas, California, and sometimes Utah and Colorado	a state in the western United States on the Pacific; the 3rd largest state; known for earthquakes