Scientists call them "natural satellites". The rest of us normally prefer to refer to them as "moons" - particularly our own Moon, or Luna as she's known to her friends. The Moon
has some of the deepest, most complex and powerful symbolism attributed to it, and with its powers over the tide, weather, and generally lighting up the night sky, who can blame it?
These are tropes that deal with those spheroids that orbit planets and are normally visible when the sun isn't in the way... most of the time.
, This Index Earth
and Stellar Index