The term 'Galilean Moon' refers to the four largest moons of Jupiter, which were discovered by Galileo Galilei in 1610. These moons are named Europa, Ganymede, Io, and Callisto. Europa is the smallest of the four moons and is composed of a rocky core surrounded by an icy crust. It is believed to have a subsurface ocean, making it one of the most promising places in the Solar System to search for extraterrestrial life.