The Collaborative International Dictionary of English v.0.48:

Thousandth \Thou"sandth\, a.
   1. Next in order after nine hundred and ninty-nine; coming
      last of a thousand successive individuals or units; -- the
      ordinal of thousand; as, the thousandth part of a thing.
      [1913 Webster]

   2. Constituting, or being one of, a thousand equal parts into
      which anything is divided; the tenth of a hundredth.
      [1913 Webster]

   3. Occurring as being one of, or the last one of, a very
      great number; very small; minute; -- used hyperbolically;
      as, to do a thing for the thousandth time.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Thousandth \Thou"sandth\, n.
   The quotient of a unit divided by a thousand; one of a
   thousand equal parts into which a unit is divided.
   [1913 Webster] Thowel

WordNet (r) 3.0 (2006):

thousandth
    adj 1: the ordinal number of one thousand in counting order
           [syn: thousandth, 1000th]
    n 1: position 1,000 in a countable series of things
    2: one part in a thousand equal parts [syn: one-thousandth,
       thousandth]