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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 ?
Caption: Atomic clocks are so precise that physicists can measure time to the quintillionth ([10.
The Agilent instrument is sensitive down to the attomole (one quintillionth, or 10 to the negative 18 power of a mole) range, and offers better than two parts-per-million mass accuracy, which gives scientists high confidence in their data and helps them easily identify the compounds they find.
The Large Electron-Positron Collider operational in 1989 at CERN laboratories near Geneva, Switzerland, can discern particles as small as 10 quintillionths of a meter.