Definitions
from The American Heritage® Dictionary of the English Language, 4th Edition
 n. The cardinal number equal to 1018.
 n. Chiefly British The cardinal number equal to 1030.
from Wiktionary, Creative Commons Attribution/ShareAlike License
 n. A billion billion: 1 followed by eighteen zeros, 1018.
 n. A million quadrillion: 1 followed by 30 zeros, 1030.
 n. Any very large number, exceeding normal description.
from The Century Dictionary and Cyclopedia
 Noting a quintillion; the cardinal numeral corresponding to a quintillion: strictly a collective noun. See hundred, a.
 n. In the English notation, the fifth power of a million, a unit followed by thirty ciphers; in the French notation, used generally in the United States, the sixth power of one thousand, a unit followed by eighteen ciphers.
from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.
 n. the number that is represented as a one followed by 18 zeros
Etymologies
Examples

One quintillion is equal to one billion billion, written as a one followed by 18 zeroes.

(An exabyte is 1 quintillion bytes; a quintillion is the number 1 followed by 18 zeroes.)
Energy News  Energy Technology  Energy Business  Energy and the Environment

Professor MORLEY DAVIDSON (Department of Mathematical Sciences, Kent State University): A quintillion is a billion, billion.

And yes, "quintillion" is a number so large, it almost seems madeup.

Then, the super prominence snapped, releasing a quintillion tons of plasma in a conical plum headed toward the Arcturian homeworld at nearly the speed of light.
365 tomorrows » Patricia Stewart : A New Free Flash Fiction SciFi Story Every Day

Mr. Skaugen noted that the acquisition will help Intel achieve "exascale" computing performance by 2018, or a quintillion operations per second.

By 2018, Japan, the U.S. and China are targeting the development of supercomputer capable of doing 1 quintillion 1 million trillion calculations per second.

"The odds of it happening are almost infinitesimally small, or precisely 1 in 2,432,902,008,176,640,000 roughly two and a half quintillion to one," begins Guy Griffiths in an email that we wished had arrived in the Knowledge inbox before we went through the past 120 years of English football tables.
Has a league ever finished in alphabetical order?  The Knowledge

Every day, people create the equivalent of 2.5 quintillion bytes of data from sensors, mobile devices, online transactions, and social networks; so much that 90 percent of the world's data has been generated in the past two years.
Arnal Dayaratna: IBM Releases Big Data Software On SmartCloud; Cognos for iPad

Most of this radioactive cesium will end up in the Pacific Ocean and will be enormously diluted in the 200 quintillion gallons of water there.
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