salad greens vs lettuce

salad greens and lettuce both are nouns.

Word	Noun	Adjective	Verb	Adverb
salad greens	Yes	No	No	No
lettuce	Yes	No	No	No

As nouns, lettuce is a hyponym of salad greens; that is, lettuce is a word with a more specific, narrower meaning than salad greens:

Other hyponyms of salad greens include celtuce, chicory, curly endive, chicory escarole, endive, escarole, cress, salad burnet.

salad greens (noun)	lettuce (noun)
greens suitable for eating uncooked as in salads	leaves of any of various plants of Lactuca sativa
	any of various plants of the genus Lactuca
	informal terms for money