Isothermal refers to a process or condition where the temperature remains constant. In thermodynamics, this means that during a change, such as the expansion or compression of a gas, the heat added or removed is balanced perfectly to keep the temperature steady. This concept is crucial in understanding how energy transfers occur without changing the thermal state of a system. An example of an isothermal process is the behavior of a gas in a piston. When the gas expands slowly, it absorbs heat from its surroundings, maintaining a constant temperature. This principle is often applied in various scientific and engineering fields, including refrigeration and heat engines.