metre vs metrical unit

metre and metrical unit both are nouns.

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

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

metre: (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 metre include catalexis, scansion, common measure, common meter, foot, metrical foot.

metre (noun)	metrical unit (noun)
rhythm as given by division into parts of equal duration	(prosody) a group of 2 or 3 syllables forming the basic unit of poetic rhythm
(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)