chiliad

n. group of 1000; millennium, time period of 1000 years; positive whole number which is the product of 10 and 100; millennium

1000 (one thousand) is the natural number following 999 and preceding 1001. The letter A does not appear in the English spelling of any number lower than "one thousand".
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Noun
1. the cardinal number that is the product of 10 and 100
(synonym) thousand, one thousand, 1000, M, K, G, grand, thou, yard
(hypernym) large integer

(n.)
A thousand; the aggregate of a thousand things; especially, a period of a thousand years.
  

A period of one thousand years