Who's a Gadol Ha'Dor?

The term "gadol ha'dor" is used freely, but what does it mean? Literally, it means great one of the generation, or the greatest of the generation.  How does one earn this title? It seems rather arbitrary.

Some people are referred to as a gadol ha'dor due to their knowledge, but then there are others who have the same or greater knowledge, but are not referred to as a gadol or gadol ha'dor.  There are some people who are referred to as a gadol ha'dor due to their position, but then there are others who have no official position who are also referred to as the gadol ha'dor.  And there are those with official positions who are not referred to as the gadol ha'dor! So if it's not knowledge nor position, what is it? Nobody is voted as gadol ha'dor.  It seems to happen by consensus by a certain number of people, although that does not mean that A's gadol is necessarily B's gadol.  That's why it seems random.

While listening to a lecture, I heard the following definition for a different term, that of "Nasi Ha'Dor."  The speaker said a Nasi of the generation takes responsibility for every Jew.  Well, that makes it very easy to see who is not a Nasi.  Not even ardent followers of those individuals called gedolei ha'dor would claim that their gadol takes responsibility for every Jew.  But that's if they're pressed against the wall. When not pressed against the wall, and when making grandiose claims that nobody is going to dispute, they say things like Rabbi X was/is the leader of all Klal Yisrael.  It makes me want to pipe up and ask: You mean even those Jews in America and Amsterdam and Finland and Bolivia and ... who never heard of him? In what way was this Rabbi X the leader of these Jews?

But I never get to ask, so I can't tell you what their answer might be.