System and Surroundings
Definition :
Thermodynamics focuses on the study of macroscopic systems, which may interact with other systems and their surroundings. The surroundings also referred to as the environment, reservoir, or bath. Typically have an extremely large scale compared to the system. While a system may experience changes in its state due to this interaction, the state of the bath remains essentially unaffected due to its significantly larger size.
Example :
For example, a thermometer[1] (the system) measuring a body's temperature interacts with the body, which acts as the thermal reservoir.