1. [n]  one part in a million
2. [n]  position 1,000,000 in a countable series of things
3. [adj]  the ordinal number of one million in counting order

MILLIONTH is a 9 letter word that starts with M.

1. \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.
   
   

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