The gigabyte is an SI-multiple of the unit byte for digital information storage. The prefix giga means 109, therefore 1 gigabyte is 1000000000bytes. The unit symbol for the gigabyte is GB or Gbyte, but not Gb (lower case b) which is used for the gigabit.
Historically, the term has also been used in some fields of computer science and information technology to denote the gibibyte, or 1073741824 (10243, 230) bytes. The Institute of Electrical and Electronics Engineers (IEEE) defined the unit accordingly for the use in power switchgear. In 2000, however, IEEE adopted the International Electrotechnical Commission (IEC) recommendation, which uses the metric prefix interpretation.
Today the usage of the unit gigabyte is still ambiguous: its value may depend on the context of usage. When referring to disk storage capacities it usually means 10003 bytes. This also applies to data transmission quantities over telecommunication circuits, as the telecommunications and computer networking industries have always used the SI prefixes with their standards-based meaning. When referring to RAM sizes it most often (see binary prefix adoption) has a binary interpretation of 10243 bytes, i.e. as an alias for gibibyte. File systems and software often list file sizes or free space in some mixture of SI units and binary units; they sometimes use SI prefixes to refer to binary interpretation - that is using a label of gigabyte or GB for a number computed in terms of gibibytes (GiB), continuing the confusion.
In order to address this confusion, the International Electrotechnical Commission has been promoting the use of the term gibibyte for the binary definition. This position is endorsed by other standards organizations including the IEEE, the International Committee for Weights and Measures (CIPM) and the U.S. National Institute of Standards and Technology (NIST), but the binary prefixes have seen limited acceptance. The JEDEC industry consortium continues to recommend the IEEE 100 nomenclature of using metric prefixes in binary interpretation for memory manufacturing designations.