The Collaborative International Dictionary of English v.0.48:

Decillionth \De*cil"lionth\, a.
   Pertaining to a decillion, or to the quotient of unity
   divided by a decillion.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Decillionth \De*cil"lionth\, n.
   (a) The quotient of unity divided by a decillion.
   (b) One of a decillion equal parts.
       [1913 Webster]