The term celestials was originally used for beings that dwell in the heavens (like angels) but is also used these days for beings that live in higher or other dimensions or beyond space and time. Advanced beings like Ascended Masters, as well as beings or civilizations that have reached a state of consciouness / density where they have transcended our 3D spacetime continuum are considered celestials.

Extraterrestrial civilizations like the Arcturians or the RA are examples of celestial civilizations.

The ascended masters of the Great White Brotherhood are also celestials.

See also: Density, Ultraterrestrial, and Higher Dimensional