Final answer:
A Galilean moon is one of the four largest moons of Jupiter: Io, Europa, Ganymede, and Callisto, discovered by Galileo Galilei.
Step-by-step explanation:
A Galilean moon refers to one of the four largest moons of Jupiter, discovered by Galileo Galilei in 1610. These moons are named Io, Europa, Ganymede, and Callisto. These satellites were significant in the history of astronomy as they provided clear evidence against the geocentric model, which stated that all celestial bodies must revolve around the Earth.
Especially noteworthy is Callisto, an ancient and primitive world, roughly 2 million kilometers away from Jupiter, where it orbits in 17 days, mirroring the rotation period which means it always shows the same face to Jupiter.
The exploration of these moons, including flybys by spacecraft such as Galileo and Juno, has greatly expanded our understanding of the Jovian system and the diversity of planetary bodies in our solar system.