What is an angel

(Archangel Michael: egg-tempera and gold leaf--Egyptian Coptic style)

I think that Angels exist--but I also think that there are no such things as angels.
That might sound contradictory.
The word 'angel' does not name a kind of thing--it means 'messenger', and refers to non-human intelligences that communicate (give messages) to humans, whatever those messenger-beings actually are.
In themselves they seem to be unimaginable, yet we imagine them and relate to them through images.
But we would be mistaken to take the images, or even our concepts, too literally.
According to Thomas Aquinas, the word 'angel' is not the name of species or genus--that is, 'angel' is not like our words 'dog' or 'animal'; there is no kind or species of creature in the universe of the nature 'angel'.  Rather there are whole worlds of other conscious and intelligent beings in the universe.  We become aware of them as they act on us or communicate with us.  And so we name them 'the messengers'--but that does not say what they are or that they are all one kind of thing.