metrical unit vs iamb

metrical unit and iamb both are nouns.

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

As nouns, iamb is a hyponym of metrical unit; that is, iamb 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
iamb: a metrical unit with unstressed-stressed syllables

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

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