You should be aware that the word “angel” denotes a function rather than a nature. Those holy spirits of heaven have indeed always been spirits. They can only be called angels when they deliver some message. Moreover, those who deliver messages of lesser importance are called angels; and those who proclaim messages of supreme importance are called archangels.
Whenever some act of wondrous power must be performed, Michael is sent, so that his action and his name may make it clear that no one can do what God does by his superior power.
-- Pope Saint Gregory the Great from a homily
