Giga- (symbol: G) is a prefix in the SI system of units denoting 109, or 1,000,000,000. 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×". Giga- comes from the Greek γίγας, meaning 'giant'.
When referring to computing information units, such as gigabit or gigabyte, giga- can sometimes mean 1,073,741,824 (230), (Though such use is incorrect) and is better used only to denote strictly 1,000,000,000 (109). Any ambiguity is best resolved from context. The binary prefix gibi- has been standardized for 230, while reserving giga- exclusively for 109, to resolve this ambiguity, but has yet to achieve widespread usage. See binary prefix.
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 [g] was originally intended as the pronunciation. Self was unable to ascertain at what point the alternative pronunciation came into occasional use, but as of 1995 it had died out.