What best defines an isothermal process?

An isothermal process is defined by maintaining a constant temperature throughout the entire process. In thermodynamics, this means that although other properties, such as pressure and volume, may change, the temperature of the system remains fixed. When a system is undergoing an isothermal process, any heat added to the system is used to do work or to change the volume of the gas, but does not affect its temperature. This characteristic is particularly significant in the context of ideal gases, where the relationship between pressure, volume, and temperature can be described by the ideal gas law.

In contrast, the other options refer to different types of thermodynamic processes: constant pressure describes isobaric processes, constant volume refers to isochoric processes, and no change in pressure indicates an isobaric or potentially isochoric scenario depending on the system's constraints. Thus, the essence of an isothermal process is specifically about temperature stability, making the correct definition centered around the constancy of temperature.