meter vs metrical unit

meter and metrical unit both are nouns.

Word	Noun	Adjective	Verb	Adverb
meter	Yes	No	Yes	No
metrical unit	Yes	No	No	No

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

meter: (prosody) the accent in a metrical foot of verse
metrical unit: (prosody) a group of 2 or 3 syllables forming the basic unit of poetic rhythm

Other hyponyms of meter include catalexis, scansion, common measure, common meter, foot, metrical foot.

meter (noun)	metrical unit (noun)
any of various measuring instruments for measuring a quantity	(prosody) a group of 2 or 3 syllables forming the basic unit of poetic rhythm
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)	metrical unit (verb)
measure with a meter
stamp with a meter indicating the postage