MSW: Modified Sine Wave. Correctly, Modified Square Wave. Refers to an inverter whose output waveform is incremental square-wave 'steps' that 'mimic' a sine wave. While usually less expensive than Sine Wave inverters, they have limitations in that some devices do not work properly or at all. Some refer to these Sine Wave inverters as True Sine Wave (TSW) or Pure Sine Wave (PSW), but that is a bit of a misnomer as the waveform is not true or pure.