Digital Multimeter, a multipurpose electrical measuring device.
Device used to measure voltage, current, resistance and other electrical quantities. Very useful for troubleshooting electrical problems, as well as preparing a locomotive for decoder installation. They may be handheld or a bench top model. The measurement is displayed digitally instead of using a needle indicating a value on a scale, as would have been seen with an analog VTVM.
Originally they were called an AVO meter (amps/volts/ohms) by their inventor. Later copies were called a multimeter, as it had multiple uses.
Note: They are not useful for measuring DCC voltages. They are designed to measure low frequency AC voltages, and present that information as the Root Mean Square Voltage. (RMS is used to represent the equivalent Direct Current heating value of AC Power). Since the DCC signal is a high frequency digital waveform, the resulting readings will be very confusing.