One of the mysteries of the English language finally explained.
How many is a billion?
In British English, a billion used to be equivalent to a million million (i.e. 1,000,000,000,000), while in American English it has always equated to a thousand million (i.e. 1,000,000,000). British English has now adopted the American figure, though, so that a billion equals a thousand million in both varieties of English.
The same sort of change has taken place with the meaning of trillion. In British English, a trillion used to mean a million million million (i.e. 1,000,000,000,000,000,000). Nowadays, it's generally held to be equivalent to a million million (1,000,000,000,000), as it is in American English.
The same evolution can be seen with quadrillion and quintillion. In British English, a quadrillion used to mean a thousand raised to the power of eight (1024), and is now understood to be a thousand raised to the power of five (1015). A quintillion, in British English, used to mean a million raised to the power of five (1030), and is now most commonly held to be a thousand raised to the power of six (1018).
Even higher are sextillion, septillion, octillion, nonillion, and decillion, some of which are not common enough to be included in OxfordDictionaries.com yet.
Other terms follow the same linguistic pattern (ending with -illion) but do not refer to precise numbers. These include jillion, zillion, squillion, gazillion, kazillion, bajillion, and bazillion. All of these words are used informally to refer to an extremely or indefinitely large number.
See other Vocabulary Questions.
Take a look at some differences between British and American English.
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