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.
When referring to HardDisk capacities it usually means 1,000 MebiBytes? (therefor mixing up the 1,024 and 1,000 based notation systems), When referring to RAM sizes it usually means a GibiByte.
It's often pronounced "gig".
