What term describes a system in which neither matter nor energy can be lost or gained?

The term that describes a system in which neither matter nor energy can be lost or gained is an isolated system. An isolated system is perfect for studying thermodynamic processes because it does not interact with its surroundings, allowing for a clear analysis of internal changes.

On the other hand, a closed system allows energy exchange but not matter exchange with its surroundings. An open system allows both energy and matter to be exchanged, making it unsuitable for the scenario described. A dynamic system typically refers to systems that are in motion or actively changing, which does not directly pertain to the conservation of matter and energy as defined in this question. Therefore, the best fit in this context is an isolated system.