They are IEC prefixes, complementary to the SI prefixes, and they only are defined for positive exponents.

  kibi : ki : 2^10
  mebi : Mi : 2^20
  gibi : Gi : 2^30
  tebi : Ti : 2^40
  ...

Conveniently, these numbers are fairly close to 10^3, 10^6, 10^9, and 10^12 respectively (but with increasing error: 2^40 is almost 10% larger than 10^12, whereas 2^10 is only 2.4% larger than 10^3).

This leads to a lot of conflation, mess, and misreporting by vendors and OS authors. Windows is an egregious example. A 1 TB (terabyte, 10^12) disk is reported in Windows as 931.7 GB. It ought to be 931.7 GiB, which is what Linux does. macOS uses the correct number but SI prefixes, so 1 TB is reported as 1 TB.