A thermos

 A thermos, also known as a vacuum flask, is a portable container designed to keep liquids hot or cold for extended periods.¹

It achieves this through several key features:

• Double-walled construction: The thermos has two walls with a vacuum between them.² This vacuum significantly reduces heat transfer through conduction and convection.³
• Insulating materials: The walls of the thermos are typically made of materials with low thermal conductivity, such as glass or stainless steel.⁴ This further minimizes heat transfer.⁵
• Tightly sealed lid: The lid of the thermos creates a tight seal, preventing heat loss through evaporation or air exchange.⁶

These features work together to create a near-perfect insulator, allowing the thermos to maintain the temperature of its contents for many hours.⁷

The term "Thermos" is often used generically to refer to any vacuum flask, even though it is a registered trademark of the Thermos brand.⁸