metrical unit vs metre

metrical unit and metre both are nouns.

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

As nouns, metre is a hypernym of metrical unit; that is, metre is a word with a broader meaning than metrical unit:

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

Other hypernyms of metrical unit include beat, cadence, measure, meter.

metrical unit (noun)	metre (noun)
(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)