business lunch vs luncheon

business lunch and luncheon both are nouns.

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

As nouns, luncheon is a hypernym of business lunch; that is, luncheon is a word with a broader meaning than business lunch:

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

Other hypernyms of business lunch include dejeuner, lunch, tiffin.

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