It is a vessel for storing hot or cold liquids so that they maintain their temperature independently of the surroundings.

The walls of the vessel consist of two thin layers of glass separated by a vacuum to reduce conduction and convection; inner surface of a glass vessel is silvered to reduce radiation; and the vessel is stoppered to prevent evaporation.

It was devised around 1872 by Sir James Dewar and is also known by its first tradename Thermo flask.