metrical foot vs iambus

metrical foot and iambus both are nouns.

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

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

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

metrical foot (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