1000 vs one thousand

1000 and one thousand both are nouns.

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

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

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

Other synonyms of 1000 include k, m, thousand.

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

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

Other synonyms of 1000 include chiliad, G, grand, K, M, thou, thousand, yard.

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

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