A proposed unit of data equal to 10^27 bytes. A brontobyte is 1000^9 bytes or 1000 yottabytes.
"Bronto-" is not an official prefix and the term brontobyte is generally attributed to the IBM Dictionary of Computing.
One brontobyte would be enough data to store a three-dimensional map of the Earth with one byte for each voxel of a one-centimetre grid.
[Where did IBM get it from?]