metrical unit vs iambus

metrical unit and iambus both are nouns.

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

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

metrical unit: (prosody) a group of 2 or 3 syllables forming the basic unit of poetic rhythm
iambus: a metrical unit with unstressed-stressed syllables

Other hyponyms of metrical unit include dactyl, iamb, anapaest, anapest, amphibrach, trochee, spondee, dibrach, pyrrhic, iambic trimeter.

metrical unit (noun)	iambus (noun)
(prosody) a group of 2 or 3 syllables forming the basic unit of poetic rhythm	a metrical unit with unstressed-stressed syllables