zucchini vs vegetable marrow

zucchini and vegetable marrow both are nouns.

Word	Noun	Adjective	Verb	Adverb
zucchini	Yes	No	No	No
vegetable marrow	Yes	No	No	No

As nouns, vegetable marrow is a hypernym of zucchini; that is, vegetable marrow is a word with a broader meaning than zucchini:

zucchini: marrow squash plant whose fruit are eaten when small
vegetable marrow: any of various squash plants grown for their elongated fruit with smooth dark green skin and whitish flesh

Other hypernyms of zucchini include marrow, marrow squash.

zucchini (noun)	vegetable marrow (noun)
small cucumber-shaped vegetable marrow; typically dark green	large elongated squash with creamy to deep green skins
marrow squash plant whose fruit are eaten when small	any of various squash plants grown for their elongated fruit with smooth dark green skin and whitish flesh