Giga-

Giga ( or ) is a unit prefix in the metric system denoting a factor of a (short-form) billion (109 or 1 000  000  000). It has the symbol G.

Giga is derived from the Greek word γίγας, meaning "giant". The Oxford English Dictionary reports the earliest written use of giga in this sense to be in the Reports of the IUPAC 14th Conference in 1947: "The following prefixes to abbreviations for the names of units should be used: G giga 109×".

When referring to information units in computing, such as gigabyte, giga may sometimes mean 1 073  741  824 (230), although such use is inconsistent, contrary to standards and has been discouraged by the standards organizations. The inconsistency is that gigabit is never (or very rarely) used with the binary interpretation of the prefix, while gigabyte is sometimes used this way. The binary prefix gibi has been adopted for 230, while reserving giga exclusively for the metric definition.

Pronunciation
In English, the initial g of giga can be pronounced (a hard g as in giggle), or  ( (a soft g as in giant, like a j sound, which shares its Greek root).

This latter pronunciation was formalized within the United States in the 1960s and 1980s with the issue by the US National Bureau of Standards of pronunciation guides for the metric prefixes. A prominent example is found in the pronunciation of gigawatts in the 1985 movie Back to the Future.

According to the American writer Kevin Self, a German committee member of the International Electrotechnical Commission proposed giga as a prefix for 109 in the 1920s, drawing on a verse by the humorous poet Christian Morgenstern that appeared in the third (1908) edition of Galgenlieder (Gallows Songs). This suggests that a hard German was originally intended as the pronunciation. Self was unable to ascertain at what point the alternative pronunciation came into occasional use, but claimed that as of 1995 it had died out.

In 1998, a poll by the phonetician John C. Wells found that 84% of Britons preferred the pronunciation of gigabyte starting with (as in gig), 9% with  (as in jig), 6% with  (guy), and 1% with  (as in giant).

Common usage

 * gigabyte—for instance, for hard disk capacity, 120 GB = 120 000  000  000  bytes; for file sizes 1 GB is often 1  073  741  824 or 230 bytes, more properly termed a gibibyte (GiB) to eliminate ambiguity. The difference is rooted in base 10 and base 2 enumeration, and is also the reason why hard drives advertised as a certain capacity have less than that capacity when installed (as hard drives are advertised in gigabytes or terabytes (base 10) whereas computers format data in gibibytes or tebibytes (base 2, powers of 2). See Hard disk units of storage capacity.
 * gigahertz—clock rate of a CPU, for instance, 3 GHz = 3 000  000  000  Hz
 * gigabit—bandwidth of a network link, for instance, 1 Gbit/s = 1 000  000  000  bit/s.
 * gigayear or gigaannum—one billion (109) years, sometimes abbreviated Gyr, but the preferred usage is Ga.