Isolated System
An isolated system is one that does not interact with its surroundings and does not exchange energy or matter with its surroundings. The total energy of an isolated system remains constant. In other words, no heat or work is exchanged between the system and its surroundings.

Explanation
Hot coffee stored in a well-insulated thermos flask is an example of an isolated system because of the following reasons:

Thermos Flask
A thermos flask is a well-insulated container that is designed to minimize heat transfer between the contents of the flask and the surrounding environment. It consists of a double-walled container with a vacuum between the walls, which helps to prevent heat transfer through conduction and convection.

Insulation
The insulation of the thermos flask further reduces heat transfer through radiation. The inner wall of the flask is coated with a reflective material that reflects thermal radiation back into the flask, preventing it from escaping. This insulation helps to maintain the temperature of the hot coffee for an extended period.

No Heat Exchange
Since the thermos flask is well-insulated, there is minimal heat exchange between the hot coffee and the surroundings. The hot coffee retains its temperature for a longer duration due to the reduction in heat transfer to the outside environment. This lack of heat exchange makes the system isolated.

No Exchange of Matter
In addition to heat exchange, an isolated system also does not exchange matter with its surroundings. The thermos flask prevents the exchange of coffee with the outside environment, ensuring that there is no matter transfer.

Conclusion
Hot coffee stored in a well-insulated thermos flask is an example of an isolated system because it does not exchange heat or matter with its surroundings. The thermos flask's insulation minimizes heat transfer, allowing the hot coffee to retain its temperature for an extended period without any external influence.