Nothing. Both terms are used for describing drive inverters. They are prominently being used interchangeably, depending on the manufacturer of the country. (For Mitsubishi FA, the term VFD is mostly used.).
VSD (Variable Speed Drive) refers to the motor shaft rotational speed (RPM). VFD (Variable Frequency Drive) refers to the inverter drive frequency (Hz) output. However, since the motor rotational speed is directly proportional to the inverter frequency output. these names (acronyms) are used interchangeably. (VFDs also control motor rotational speed, and VSDs also have a range of drive frequency output).
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