What does celestial mean in 'celestial bodhisattva'. I'm aware of the definition of the term "celestial", but wondered if it meant that the bodhisattva was not born on earth, so that e.g. tantric masters are not celestial beings.
In Tibetan Buddhism they recognize a class of beings that we can call "latent informational energy entities". In modern language they can be explained as long-term memes or archetypes inhabiting the sphere of knowledge and capable of producing activity through that sphere.
This includes spirits, guardians, dakinis, and celestial bodhisattvas.
They are not organisms in our sense of the word, but they are acting entities in an abstract sense, having a certain identity, relationships with the world of people, and even something like attitude or perspective, that allows them to respond or react to human acts.
You could say that this is a kind of animism towards religious archetypes, and you would be right, except that our own system of political parties, corporations etc. is in essence based on the same principles.
So yeah, celestial Bodhisattvas are characters, like Batman or Spiderman, that have a certain role and function in human society. As opposed to normal Bodhisattvas who are actual people.