one thousand thousand vs 1000000

one thousand thousand and 1000000 both are nouns.

Word	Noun	Adjective	Verb	Adverb
one thousand thousand	Yes	No	No	No
1000000	Yes	No	No	No

As nouns, one thousand thousand and 1000000 are synonyms defined as:

one thousand thousand and 1000000: the number that is represented as a one followed by 6 zeros

Other synonyms of one thousand thousand include meg, million.

one thousand thousand (noun)	1000000 (noun)
the number that is represented as a one followed by 6 zeros	the number that is represented as a one followed by 6 zeros