An imaginary world is a world in a fictional universe, which may be used as the whole or part of a campaign setting. Imaginary worlds are particularly common in fantasy role-playing games. For example, Oerth is the imaginary world of the Greyhawk setting for Dungeons & Dragons.
Alien planets are not usually called imaginary worlds, as science-fiction is a form of speculative fiction that purports to imagine what is possible. While there is no planet Vulcan, Star Trek places the planet in our universe. However, there is no logical inconsistency in using the term to refer to science-fiction worlds.
A world that is much like Earth but divergent in some respects is an alternate world, while the world we live in is sometimes fictionalized. Although the Earth of James Bond is part of his fictional universe and is not real, that Earth is not properly an imaginary world. It is a fictionalized Earth with imaginary elements.