Define "Blue Moon"

Every good night photographer knows the definition of the term "blue moon". It's "two full moons in a single month". Right?

WRONG!!!

I have to admit I didn't know the correct answer either, until I saw Mike Johnston's post this afternoon.

Apparently, a blue moon is defined as four full moons in a single season (which normally has only three). Read, or listen to, this article for more details about how this myth began.