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A vacuum flask is an insulated container designed to keep liquids hot or cold for extended periods by minimizing heat transfer. It matters because it allows people to transport and store beverages at their desired temperature throughout the day.
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Encyclopedic overview
The typical design of a Thermos brand vacuum flask, used for maintaining the temperature of fluids such as coffee
A vacuum flask (also known as a Dewar flask, Dewar bottle or thermos) is an insulating storage vessel that slows the speed at which its contents change in temperature. It greatly lengthens the time over which its contents remain hotter or cooler than the flask's surroundings by trying to be as adiabatic as possible. Invented by James Dewar in 1892, the vacuum flask consists of two flasks, placed one within the other and joined at the neck. The gap between the two flasks is partially evacuated of air, creating a near-vacuum which significantly reduces heat transfer by conduction or convection. When used to hold cold liquids, this also virtually eliminates condensation on the outside of the flask.
Excerpted from Wikipedia’s “vacuum flask” article, available under the CC BY-SA 4.0 licence.