It is you. You think you are gone and dead because the last person you felt the most for wrote you off, but you are not haunting anyone, you are solid.
It is them. It is a living person, a ghost, a dream that may have been reality. They are solid but they are haunting you, so they are a ghost.
It is a memory that should be left for dead but which you choose to continue to actively recall, likely late at night or when you are alone. It is a wisp of a thing that should not be given life, but you can't let go.
The content on this site is not 100% related to solid ghosts, but some of it is and that's good enough.