Arcane casters living in northern Avistan have a bond with cold. Mostly present in Irrisen, the land of eternal winter, Rimewitches are wizards, sorcerers, or witches that become so used to the harsh winter that they are able to draw power from the ice around them.
Mostly, wizards and witches become rimewitches, but sorcerers often do as well, certainly sorcerers with the cold elemental bloodline. Bards sometimes become rimewitches, as do summoners. Rimewitches are seen as potential allies to the Winter Witches of Irrisen, and as potent seers and leaders in the whole of the North.
Most rimewitches learn to draw more power from colder environments and to summon creatures fortified by the powers of ice and snow. They also gain rime-layered skin that proves harder to pierce. Ultimately, they become creatures of coldness, able to freeze people with a glance.
Sources: Frostburn, Own Stuff