Vacuum flasks, also known as thermos flasks, thermal bottles, or insulated bottles, are designed to keep hot drinks hot and cold drinks cold for extended periods. These flasks work on the principle of vacuum insulation, which minimizes heat transfer. Another synonym for vacuum flask is a thermos bottle, which comes from the Thermos brand that pioneered this technology. These bottles are also referred to as double-walled flasks, as they consist of two layers of glass or stainless steel with a vacuum between them. Some other synonyms for vacuum flasks include insulated coffee mugs, thermal carafes, and hot/cold insulated bottles. Whatever you call them, these flasks are excellent for taking hot coffee or cold drinks on the go.