Etymology
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billion (n.)

1680s, from French billion (originally byllion in Chuquet's unpublished "Le Triparty en la Science des Nombres," 1484; copied by De la Roche, 1520); see bi- "two" + million. A million million in Britain and Germany (numeration by groups of sixes), which was the original sense; subsequently altered in French to "a thousand million" (numeration by groups of threes) and picked up in that form in U.S., "due in part to French influence after the Revolutionary War" [David E. Smith, "History of Mathematics," 1925]. France reverted to the original meaning in 1948. British usage is truer to the etymology, but U.S. sense is said to be increasingly common there in technical writing.

In Italian arithmetics from the last quarter of the fifteenth century the words bilione or duilione, trilione, quadrilione or quattrilione, quintilione, cinquilione, or quinquilione, sestione or sestilione, settilione, ottilione, noeilione and decilione occur as common abbreviations of due volte millioni, tre volte millione, etc. In other countries these words came into use much later, although one French writer, Nicolas Chuquet, mentions them as early as 1484, in a book not printed until 1881. The Italians had, besides, another system of numeration, proceeding by powers of a thousand. The French, who like other northern peoples, took most if not all their knowledge of modern or Arabic arithmetic from the Italians, early confounded the two systems of Italian numeration, counting in powers of a thousand, but adopting the names which properly belong to powers of a million. [Century Dictionary]

For a time in Britain gillion (1961), based on giga-, was tried as "a thousand million" to avoid ambiguity. Compare milliard.

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septillion (n.)
1680s, from sept- (see septi-) + (m)illion. Compare billion.
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gazillion (n.)

by 1984, with made-up prefix and ending from billion, trillion, etc. Perhaps based on earlier zillion.

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billionaire (n.)

1844, American English, from billion on model of millionaire. Marked "[Rare.]" in Century Dictionary (1902). The first in the U.S. likely was John D. Rockefeller, some time after World War I.

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sextillion (n.)
1680s, from Latin sext-, combining form of sex "six" (see six) + (m)illion. Compare billion. Related: Sextillionth.
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quintillion (n.)
1670s, from Latin quintus "the fifth" (from PIE root *penkwe- "five") + ending from million. Compare billion. In Great Britain, the fifth power of a million (1 followed by 30 zeroes); in U.S. the sixth power of a thousand (1 followed by 18 zeroes).
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trillion 
1680s, from French trillion, from Italian trilione; see tri- + million. In the U.S., the fourth power of a thousand (one thousand billion, 1 followed by 12 zeroes); in Great Britain, the third power of a million (one million billion, 1 followed by 18 zeroes), which is the original sense. Compare billion.
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milliard (n.)

"one thousand million," 1793, from French milliard (16c.), from million (see million) with change of suffix. A word made necessary by the double meaning of billion. It became familiar in English in news coverage of the indemnity paid by France to Germany after the war of 1870-71.

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quadrillion (n.)

1670s, from French quadrillion (16c.) from quadri- "four" (from PIE root *kwetwer- "four") + (m)illion. Compare billion. In Great Britain, the fourth power of a million (1 followed by 24 zeroes); in the U.S., the fifth power of a thousand (1 followed by 15 zeroes).

Thomas Hope, first of the family to possess the Deepdene, was the author of "Anastasius," a book of the same class as Beckford's "Vathek." In each case a millionaire (we shall soon have billionaires, trillionaires, quadrillionaires) fettered, imprisoned, by abject opulence, strove to reveal himself to the world through a romance. [Mortimer Collins, "A Walk Through Surrey," Temple Bar, August 1866]
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