A variable-frequency drive (VFD) or a variable speed drive (VSD) or adjustable speed drive (ASD), is an electronic device adjusting voltage and frequency supplied to a motor to control its speed, for precise control of motor speed and torque, optimizing performance and energy consumption in many usages.
Note: VFD and VSD aren't exactly the same.
[2] A VFD saves one's energy bill by dramatically reduce energy consumption compared to direct-on-line (DOL) operation, where a motor runs at full speed no matter the demand. Via a drive, power/fuel savings of 40% are common. At times savings are ~80%. The roll-on effect means usages of drives also reduces NOx emissions and CO2 footprint of systems that they are installed.