The pervasive definition today, which is what you have, is a blue moon is the second full moon in a calendar month. This definition was started in an article published in the March 1946 issue of Sky & Telescope. The author was referring to a calendar of blue moons given in the Maine Farmer's Almanac, and misinterpreted how the almanac determined blue moons, and simply stated that it was the second full moon in a calendar month. The error was perpetuated by Sky & Telescope as late as March 1999, but in May 1999 they published an article  that finally explained the on-going error.  The problem is, no one has caught on to the error.

there ya go, a man that knows how to admit, and correct his mistakes,  cheers mate.