item vs line item

item and line item both are nouns.

Word	Noun	Adjective	Verb	Adverb
item	Yes	No	No	Yes
line item	Yes	No	No	No

As nouns, line item is a hyponym of item; that is, line item is a word with a more specific, narrower meaning than item:

item: a distinct part that can be specified separately in a group of things that could be enumerated on a list
line item: an item in an appropriation bill

Other hyponyms of item include agenda item, incidental, inventory item, news item, place, position.

item (noun)	line item (noun)
a whole individual unit; especially when included in a list or collection	an item in an appropriation bill
an isolated fact that is considered separately from the whole
a distinct part that can be specified separately in a group of things that could be enumerated on a list
an individual instance of a type of symbol
a small part that can be considered separately from the whole

item (adverb)	line item (adverb)
(used when listing or enumerating items) also