What is the difference between a thermos flask and vacuum flask?

"Thermos flask" and "vacuum flask" are terms often used interchangeably, but they generally refer to the same type of insulated container designed to keep liquids hot or cold. The key feature of both is the vacuum insulation that minimizes heat transfer between the contents of the flask and the external environment.

However, the term "Thermos" is actually a brand name that has become synonymous with vacuum flasks in many parts of the world. In some regions, people commonly refer to any vacuum flask as a "thermos," regardless of the actual brand. The word "thermos" is derived from the Greek word "therme," meaning heat.

So, in summary, a vacuum flask is a container with double walls and a vacuum between them to minimize heat transfer, while "Thermos" is a specific brand that has become widely associated with this type of product. In casual conversation, the terms are often used interchangeably, but it's essential to recognize that "vacuum flask" is a more generic term that includes products from various manufacturers, while "Thermos" specifically refers to products from the Thermos brand.