lunch vs business lunch

lunch and business lunch both are nouns.

Word	Noun	Adjective	Verb	Adverb
lunch	Yes	No	Yes	No
business lunch	Yes	No	No	No

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

lunch: a midday meal
business lunch: lunch (usually at a restaurant) where business is discussed and the cost is charged as a business expense

lunch (noun)	business lunch (noun)
a midday meal	lunch (usually at a restaurant) where business is discussed and the cost is charged as a business expense

lunch (verb)	business lunch (verb)
take the midday meal
provide a midday meal for