Giga ( or ) is a unit prefix in the metric system denoting a factor of a short-scale billion or long-scale milliard (10⁹ or 1000000000). It has the symbol G.

Giga is derived from the Greek word γίγας (gígas), 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 Conférence Internationale de Chimie 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 1073741824 (2³⁰); this causes ambiguity. Standards organizations discourage this and use giga- to refer to 10⁹ in this context too. Gigabit is only rarely used with the binary interpretation of the prefix. The binary prefix gibi has been adopted for 2³⁰, while reserving giga exclusively for the metric definition.