A large vacuum jacketed double walled container generally made out of stainless steel that is used to transport cryogens. Cryogenic liquids are shipped and used non-pressurized in this thermally insulated containers. Dewar flasks are specifically designed to withstand rapid temperature changes and extreme differences in temperature. Flasks containing helium (used in MRI for cooling of superconducting magnets), hydrogen and other low-boiling liquid cryogens have an outer vessel of liquid nitrogen for insulation.
Dewars are also called cryostats.

Cryostats are devices for maintaining a constant low temperature (as by means of liquid helium used in MRI). They require vacuum chambers with thermal isolation.

See also Dewar, Cryoshielding and Boil off Rate.