A person who believes that they are one of the following: a reincarnated extra terrestrial, a person who is descended from extra terrestrials, or an extra terrestrial on earth. (aliens people, aliens!)
Starseeds claim to be here for many diverse reasons, ranging from to save the planet from the horrible polluting people to help human beings ascend and become 4th dimentional.
"I'm a starseed," she said, "I'm going to help all the poor humans make this world a better place."
Properly written "Star Seed." Sailor Moon reference, specifically the Stars season (year 5). A tiny, glowing crystal that represents the vital force and purity of its owner. Galaxia (the main villainess) is after these for galactic domination, but ordinary Star Seeds have very little shine and quickly turn black when separated from their owners. Said owners then become "Phage Senshi," who must be wounded critically and healed by Eternal Sailor Moon's second-tier healing magic.
Active Star Seeds do not fade to black upon removal from their hosts, and are sources of great power. As far as we know, only a planet's Senshi possess these. It is possible to live without a Star Seed, if you are given strong enough magic (as in Galaxia's bracelets). Active Star Seeds can apparently also take on different forms; Galaxia's Star Seed became a young girl who called herself ChibiChibi, for example.
Losing your Star Seed is an almost certain death sentence.
A person who has potential for greatness. This potential could be expected (such as a young prince) or discovered (such as a child prodigy). A starseed is, literally, a seed who will, could, or should, become a star.
A 3 year old child who displays a profound understanding of science, or math, or music, could be considered a starseed.