The Collaborative International Dictionary of English v.0.48:

Millionth \Mil"lionth\, a.
   Being the last one of a million of units or objects counted
   in regular order from the first of a series or succession;
   being one of a million.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Millionth \Mil"lionth\, n.
   The quotient of a unit divided by one million; one of a
   million equal parts.
   [1913 Webster]

WordNet (r) 3.0 (2006):

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