In the context of a modular synthesizer, a power supply that mounts in the case as a functional module, as opposed to an "open frame" supply mounted inside the case, or an external supply. The main advantage of a power module is easy integration: the module connects to the power bus the same as any other module, minimizing the amount of work the modular owner has to do in order to get the synth up and running. Power modules will usually also have LEDs on the panel to indicate the status of the various voltages being output by the supply. Some more sophisticated ones may include a voltmeter so the user can check to see that the voltages are within tolerance.
The main disadvantage of a power module is that it takes up space that could be used by other modules. Heat can also be an issue. Module power interconnect cables tend to not be capable of handling high currents, so it is usually not practical to offer high-capacity supplies as modules. (Higher-capacity supplies will have some mechanism of using heavier conductors to connect to the bus.)