septillionth

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

• noun The ordinal number matching the number septillion in a series.
• noun One of a septillion equal parts.

from The Century Dictionary.

• Last in a series of a septillion individuals: an ordinal numeral.
• Being one of a septillion equal portions.
• noun One of a septillion equal parts of anything.

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

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