solion

/ˈsoʊliˌɑn/
noun
  1. A hypothetical particle or entity that exists only in one's own mind or imagination, not in the external world.
    • In her novel, the main character's imaginary friend was a solion that only she could see.
    • Some scientists debate whether dark matter is a real substance or a solion created by incomplete theories.
    • The philosopher argued that the idea of a perfect circle is a solion, not something found in nature.
Antonyms