A multimeter is a measuring instrument that can measure multiple electrical properties. A typical multimeter can measure voltage, resistance, and current, in which case can be used as a voltmeter, ohmmeter, and ammeter. Some feature the measurement of additional properties such as temperature and capacitance.
An analog multimeter
1920s pocket multimeter
Avometer Model 8
A pocket-watch-style meter made in the 1930s. It can measure voltage, current, continuity and the heating element of vacuum tubes
Voltage, also known as (electrical) potential difference, electric pressure, or electric tension is the difference in electric potential between two points. In a static electric field, it corresponds to the work needed per unit of charge to move a positive test charge from the first point to the second point. In the International System of Units (SI), the derived unit for voltage is the volt (V).
Batteries are sources of voltage in many electric circuits.
Working on high voltage power lines
Multimeter set to measure voltage