The prefix "Celestial", when used in the Factions campaign, indicates a link with the spirits of Cantha. Some of these spirits are so powerful that they have almost divine status, below the gods.
Many powerful spirits of Cantha have strong affiliations with the sky and the stars, hence the name. Canthans see spirits in at least some of the star constellations and zodiac signs.
Examples:
Humans sometimes apply the term celestial to more mundane things, such as the Celestial Ministry.