octillionth

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun The ordinal number that matches the number octillion in a series.
• noun One of an octillion equal parts.

from The Century Dictionary.

• The ordinal numeral adjective corresponding to octillion; last in an ordered set of an octillion elements.
• Being one of an octillion all equal: as an octillionth part.
• noun The quotient of unity divided by an octillion.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• adjective The ordinal form of the number octillion.
• noun The person or thing in the octillionth position.
• noun One of a octillion equal parts of a whole.