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A screw-top container (coming in various designs) using a double-skinned flask holding a vacuum. A vacuum can't transfer any heat, so the flasks keep hot food hot and cold food cold.
They were invented by Sir James Dewar in the early 1890's, but commercially developed by the German glass-blowing company Burger and Aschenbrenner. They organized a competition to name the product, and the word 'thermos' was chosen - being derived from the Greek word meaning heat.