What is the exact definition of "Vus iz di nafke meene"?
"Vus is de" is Yiddish for what is the ...
Nafka mina is the Aramaic for comes out from this. If there is a difference of opinion in the Talmud on a theory, a nafka mina would be a case whose ruling would depend on the difference in theory. For example the Talmud says not to eat while traveling. Why not? One reason is "you may run out of food and starve"; the other is "it's bad for you digestion." What is a nafka mina between those two reasons? Say you're traveling along Route 66 and there's a convenience store every half mile. You certainly won't starve ... but it's still bad for your digestion.
Sir Arthur Eddington, for example, realized he could measure a nafka mina between Newtonian and Einsteinian theories of space and time.
So -- what's the nafka mina?! --"What practical difference would stem from this"?
For instance, a philosopher might get up and lecture: "some rabbis view the State of Israel as the End of the Exile Period; others view it as the Dawn of the Redemption Period." Great, but someone could then ask -- what's the nafka mina? So what? What would be an effect I'd see differently because of those two opinions?