The Collaborative International Dictionary of English v.0.48:

1000 \1000\ adj.
   1. 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

   Syn: thousand, a thousand, one thousand, m, k
        [WordNet 1.5 +PJC]

WordNet (r) 3.0 (2006):

1000
    adj 1: denoting a quantity consisting of 1,000 items or units
           [syn: thousand, one thousand, 1000, m, k]
    n 1: the cardinal number that is the product of 10 and 100 [syn:
         thousand, one thousand, 1000, M, K, chiliad,
         G, grand, thou, yard]