Ghost planet

A ghost planet was the designation James T. Kirk suggested for the Kalandan outpost, given its improbable existence. Spock rejected a supernatural explanation for the planet's inability to fit into any known planetary category.

Later, after witnessing the disappearance of (who would later turn out to be a hologram), Kirk suggested that Spock was wrong, and that it was a ghost planet after all (i.e. inhabited by a ghost, not merely mysterious).