A vacuum flask, also known as a thermos, is a container used to keep liquids hot or cold for extended periods of time. There are several synonyms for vacuum flask, including insulated bottle, thermal bottle, thermal flask, and double-walled bottle. Insulated and thermal bottles highlight the ability of the flask to retain temperature, while double-walled indicates the design of the flask. Another synonym, heat flask, describes the intended use of the container for keeping beverages hot. Regardless of what it's called, a vacuum flask is a convenient and useful item for anyone who wants to keep their drinks at the perfect temperature for hours.