There are two definitions of a Blue Moon in astronomy; both are a type of full moon. (If the moon does look blue, it’s caused by a rare type of dust in the atmosphere.)
In astronomy, a Blue Moon is a full moon that doesn’t quite fit in with the months in our calendar.
There are two different ways of calculating which full moon is a Blue Moon.
Blue Moon = The third full moon in an astronomical season with 4 full moons (versus the normal 3)
Blue Moon = The second full moon in a month with two full moons.
That being said, the correct, original definition is that a “Blue Moon” is the third full moon in an astronomical season with four full moons.
A normal year has four astronomical seasons — spring, summer, fall and winter — with three months and normally three full moons each. When one of the astronomical seasons has four full moons, instead of the normal three, the third full moon is called a Blue Moon.