Cryopreservation is the technique of freezing tissues, cells or other biological materials (say, the severed heads of people who want to be revived in some utopian future) at very low temperatures. At these low temperatires, the materials remain genetically stable and metabolically inert. Cryopreservation may involve freezers (-80 degrees Celsius), or preservation with dry ice (-79 degrees) or liquid nitrogen (-196 degrees).
Tissues being frozen in this maner are generally treated with a cryoprotectant, a substance that prevents ice crystals from forming and damaging cells.