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1. The ordinal number matching the number quintillion in a series.
2. One of a quintillion equal parts.

quin·til′lionth adv. & adj.
ThesaurusAntonymsRelated WordsSynonymsLegend:
Noun1.quintillionth - one part in a quintillion equal parts
common fraction, simple fraction - the quotient of two integers
Adj.1.quintillionth - the ordinal number of one quintillion in counting order
ordinal - being or denoting a numerical order in a series; "ordinal numbers"; "held an ordinal rank of seventh"
References in periodicals archive ?
In March of 2014 scientists who had been observing the cosmic microwave background with a microwave telescope at the South Pole announced the discovery of gravity waves that formed in less than a quintillionth of a second after the 'Big Bang'.
Caption: Atomic clocks are so precise that physicists can measure time to the quintillionth ([10.
3 attojoules (less than one quintillionth of a joule)," says researcher Min-Feng Yu.