Indian lettuce vs herb

Indian lettuce and herb both are nouns.

Word	Noun	Adjective	Verb	Adverb
Indian lettuce	Yes	No	No	No
herb	Yes	No	No	No

As nouns, herb is a hypernym of Indian lettuce; that is, herb is a word with a broader meaning than Indian lettuce:

Indian lettuce: a plant of the genus Montia having edible pleasant-tasting leaves
herb: a plant lacking a permanent woody stem; many are flowering garden plants or potherbs; some having medicinal properties; some are pests

Other hypernyms of Indian lettuce include herbaceous plant.

Indian lettuce (noun)	herb (noun)
a plant of the genus Montia having edible pleasant-tasting leaves	aromatic potherb used in cookery for its savory qualities
	a plant lacking a permanent woody stem; many are flowering garden plants or potherbs; some having medicinal properties; some are pests