voltmeter

from WordNet (r) 3.0 (2006)

voltmeter
    n 1: meter that measures the potential difference between two
         points

from The Collaborative International Dictionary of English v.0.48

Voltmeter \Volt"me`ter\, n. [2d volt + -meter.] (elec.)
   An instrument for measuring in volts the differences of
   potential between different points of an electrical circuit.
   [1913 Webster]