meter vs voltmeter

meter and voltmeter both are nouns.

Word	Noun	Adjective	Verb	Adverb
meter	Yes	No	Yes	No
voltmeter	Yes	No	No	No

As nouns, voltmeter is a hyponym of meter; that is, voltmeter is a word with a more specific, narrower meaning than meter:

meter (noun)	voltmeter (noun)
any of various measuring instruments for measuring a quantity	meter that measures the potential difference between two points
rhythm as given by division into parts of equal duration
(prosody) the accent in a metrical foot of verse
the basic unit of length adopted under the Systeme International d'Unites (approximately 1.094 yards)

meter (verb)	voltmeter (verb)
measure with a meter
stamp with a meter indicating the postage