What exactly is a moon?

How is it defined? Does it matter what size it is? How many moons are there in the solar system?

Earth is the only planet with just one moon. File picture: PA

We had a Halloween Blue Moon last week, turning our eyes skyward. We have one moon orbiting our planet, but other planets have moons too; their moons even get interesting and important-sounding names, such as Titan and lo. How are moons defined in the first place? What makes a moon a moon?

Lunar definition