1,000 MegaBytes, but commonly refers to 1,024 MebiBytes which is actually called a GibiByte. However, this distinction has not been adopted in practice, so it's often unclear what exact capacity is meant by a particular mention of GigaByte.
It's often pronounced "gig".
When referring to HardDisk capacities it does usually mean 1,000 MegaBytes. When referring to RAM sizes it usually means a GibiByte.
See also KiloByte/KibiByte, MegaByte/MebiByte, GibiByte, TeraByte.
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