A power-saving mode in which you can place a laptop or other power managable device (if you have OS or hardware support for it). Unlike sleep, which uses some power over a long period of time, hibernate shuts the machine entirely off.

Hibernation is typically writing all of a machine's memory to disk, flipping a flag in the boot sector, and shutting off. When the machine turns back on, it will write all of the memory from the disk, back into memory, and resume processing. I know that Windows 2000 has support for this ( I am not sure about MacOS or Linux)

This works prefectly on my laptop, and is great for long trips. The only problem with it for me is that my clock tends to get way out of sync, but the NT domain fixes that.