Exa (símbolo E) es un prefijo del Sistema Internacional que indica un factor de 1018 (un trillón). Adoptado en 1975, viene del griego έξα («éxa») que significa seis (como «hexa-»), pues equivale a 10006. En informática, exa puede significar 260, en vez de 1 000 000 000 000 000 000, especialmente cuando se utiliza como prefijo de byte (exabyte).

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Exa is a decimal unit prefix in the metric system denoting 1018 or 1000000000000000000. It was added as an SI prefix to the International System of Units (SI) in 1975, and has the unit symbol E. Exa comes from the Ancient Greek ἕξ, used as a prefix ἑξά-, meaning six (like hexa-), because it is equal to 10006. Examples: \n* The total storage needed by Google Mail as of April 2012, ignoring backups and compression, is more than an exabyte (10,240 megabytes of storage per user multiplied by an estimated 260 million users). \n* 1 EeV = 1018 electronvolts = 0.1602 joule \n* 1 exasecond is approximately 32 billion years \n* 1 exametre is approximately 110 light years \n* 0.43 Es ≈ the approximate age of the Universe \n* 1.6 Em—172 ± 12.5 light years—Diameter of Omega Centauri (one of the largest known globular clusters, perhaps containing over a million stars)

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