A thermos flask or a thermos is a bottle designed to keep a warm substance warm and cold things cold. It maintains the temperature of the contents, the content could be in liquid form or it could be in solid form. The container usually has a cylindrical opening with the inner surface and the outer surface separated by a vacuum. This is what maintains the liquid or solid within the flask at the temperature it is. The vacuum doesn't allow the heat or cold to conduct outside the thermos.

The thermos becomes an excellent option for storing gases as nitrogen in a laboratory that would liquefy if they are exposed to room temperature. This way, the need for expensive refrigerators is met with vacuum flasks of relatively less cost. The thermos was invented by the Scottish physicist and chemist, Sir James Dewar. They are also called Dewar flasks. They were brought into the commercial world by a German company, Thermos GmbH in 1904.

The material of vacuum flasks could vary from being a metal to plastic or glass. Reflective material like glass or silver is coated on the interiors to counter relative heat loss. The Dewar flasks used silver. All of them have at least one opening usually made of cork or any other insulating material; the cork over the opening usually forms the outlet for the heat that is lost. It is capable of keeping liquids cool for 24 hours, while it could maintain the heat of liquids for 8 hours.