metrical foot vs iamb

metrical foot and iamb both are nouns.

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

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

metrical foot: (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 foot include dactyl, iambus, anapaest, anapest, amphibrach, trochee, spondee, dibrach, pyrrhic, iambic trimeter.

metrical foot (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