one thousand vs 1000

one thousand and 1000 both are nouns.

Word	Noun	Adjective	Verb	Adverb
one thousand	Yes	Yes	No	No
1000	Yes	Yes	No	No

As adjectives, one thousand and 1000 are synonyms defined as:

one thousand and 1000: denoting a quantity consisting of 1,000 items or units

Other synonyms of one thousand include k, m, thousand.

As nouns, one thousand and 1000 are synonyms defined as:

one thousand and 1000: the cardinal number that is the product of 10 and 100

Other synonyms of one thousand include chiliad, G, grand, K, M, thou, thousand, yard.

one thousand (noun)	1000 (noun)
the cardinal number that is the product of 10 and 100	the cardinal number that is the product of 10 and 100

one thousand (adjective)	1000 (adjective)
denoting a quantity consisting of 1,000 items or units	denoting a quantity consisting of 1,000 items or units