leftovers vs solid food

leftovers and solid food both are nouns.

Word	Noun	Adjective	Verb	Adverb
leftovers	Yes	No	No	No
solid food	Yes	No	No	No

As nouns, solid food is a hypernym of leftovers; that is, solid food is a word with a broader meaning than leftovers:

leftovers: food remaining from a previous meal
solid food: any solid substance (as opposed to liquid) that is used as a source of nourishment

Other hypernyms of leftovers include food.

leftovers (noun)	solid food (noun)
food remaining from a previous meal	any solid substance (as opposed to liquid) that is used as a source of nourishment