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n. In electricity, a direct-reading instrument for the measurement of small differences of potential, in which the scale is graduated to read in thousandths of a volt.

n. sensitive voltmeter that can measure voltage in millivolts

"I usually use my Boonton RF millivoltmeter, which has way more sensitivity than a regular diode detector RF probe with a DVM or VTVM, although -70 dBm sensitivity isn't needed, either."

voltmeter