quintillion

A quintillion is a large number with two common definitions depending on the numeric scale in use. In the contemporary short scale, used in the United States and increasingly elsewhere, a quintillion equals 1,000,000,000,000,000,000 (10^18). The term is formed from quint- meaning five, combined with million, following the standard naming pattern for large numbers.

Historically, some European languages used the long scale, where a quintillion is 1 × 10^30. In that system, each new name after a million represents a factor of one million (10^6), so billion is 10^12, trillion 10^18, quadrillion 10^24, and quintillion 10^30. Over time, many English-language and international references have adopted the short scale, but long-scale usage persists in older texts and in some non-English contexts, which can lead to ambiguity.

In relation to other large numbers, the short scale sequence around quintillion is: thousand (10^3), million (10^6), billion (10^9), trillion (10^12), quadrillion (10^15), quintillion (10^18). In the long scale, the corresponding sequence places the quintillion at 10^30, with intermediate names shifted accordingly. For precise communication, especially in scientific or mathematical contexts, exponent notation (10^18) or the SI prefix exa- (10^18) is often preferred over verbal names.