Source : Webster's Revised Unabridged Dictionary (1913)

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.

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

Source : WordNet®

millionth
     adj : 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 [syn: {one-millionth}]