@aDaikon celestial have 2 meaning
1st- something formed in outer space without interference of something not formed in outer space.
If you want to refer a creature celestial then it means it is formed naturally in outer space without any other interference (outer space is a extreme environment) which means that is born in an environment where there is barely any fluid, organic materials to sustain itself, not to mention it doesn't even have a atmosphere to stop him from disintegrating into atoms and it somehow sustains it's life which means it has to have.
1) not dependent on chemical reaction for energy.
2)have super hard skin.
3) can survive in coldest (between galaxies) and hotest (inside stars or near a star) environment without any problem.
4)have a magnetic field generating from inside to protect from cosmic rays.
5) have some kind of way to nullify gravity.
6)can move nullify his mass to move.
And that kind of creatures for us who are not celestial can be only termed as a super creature or even gods because they can literally obliterate law of physics and destroy the universe.
2nd- it means heavenly and if you add race in it then it means heavenly race.