What constitutes a joke, and what makes it funny?

One of my recent attempts. imgflip.com/i/7lq2w9

My wife insists that as long as at least 1 person finds something funny, it is a joke. This would mean that by definition all jokes are funny, even if the only person who thought so was the person telling it. If a comic was trying out new material that they weren't sure was funny, it would be left to audience to determine whether or not that material constituted a joke. Not sure I agree with this assertion but I stress tested the shit out of it just the same and came to some interesting conclusions.

Take the hypothetical scenario of someone coming up with comedic material that they themself don't find funny and testing it on someone else. Let's say that at the time the other person doesn't find it funny either, but days later their mood improves and upon reflection the material suddenly strikes them as funny. That would mean the material didn't become a joke until considerably after the fact. It would also mean that as long as only 1 person thought the material funny, its classification as a joke would be contingent on that person's mood.

Consider a comic using material they KNOW will bomb, simply to troll the audience. (Norm Macdonald and Andy Kaufman loved this kind of thing.) That situation is itself a joke, by the performer and for the performer, and at the audience's expense. Essentially a prank.

My favorite example is a comic delivering a mediocre routine that isn't really funny, and hoping the audience is drunk enough to let them get away with it. They don't. The set bombs and the comic chokes hard and goes down in flames. No laughs at all--the audience is annoyed and the comic is humiliated. No laughs that is except for another comic performing that night, witnessing this situation as it unfolds, recognizing what's happening, and laughing at how perfectly awful the circumstance is for all parties involved. HE found it funny. Bam, the whole set was a joke!