An expansion or compression which occurs without heat transfer to or from the gas is:

The term "adiabatic" specifically refers to a process in which there is no heat transfer into or out of a system. In the context of thermodynamics, when a gas expands or compresses adiabatically, it does so without any exchange of heat with its surroundings. This means that all the energy change in the system results from work being done on or by the gas, which typically leads to changes in temperature.

This process is critical in understanding how gases behave under varying conditions and is a fundamental concept in power engineering. For example, in an ideal gas undergoing an adiabatic process, the relationship between pressure, volume, and temperature can be described using specific equations derived from the laws of thermodynamics.

The other options describe processes that involve thermal exchange or different states of the system but do not align with the concept of no heat transfer. Constant temperature refers to isothermal processes where temperature remains steady, constant volume implies no change in volume occurs during the process, and polytropic refers to processes that can involve heat transfer depending on the specific value of a polytropic index.