The year 1000 of the Gregorian Calendar was the last year of the 10th century as well as the last year of the first millennium. AD 1000 was a leap year starting on Monday.

denoting a quantity consisting of one more than nine hundred ninety nine and one less than one thousand and one; -- representing the number one thousand as Arabic numerals

denoting a quantity consisting of 1,000 items or units

1000 (one thousand) is the natural number following 999 and preceding 1001.

the cardinal number that is the product of 10 and 100