A 'proper term of intellect' is a form apart from matter, as 'brightness,' 'clearness,' 'lucidity,' apart from 'this bright button,' 'this clear sky.' The separation of the form apart from matter is either physical or logical. Where the separation is logical only, the form cannot really exist except in matter; and from matter the human intellect gathers it by abstraction and generalisation. The angel somehow gathers the same form without having to study the matter in which it resides. But when the form is physically distinct from matter, - when it subsists by itself, -- such an immaterial, subsisting form lives and understands: it is an angel. An angel, according to St Thomas, is a personified form, quality, or attribute: what attribute exactly, it is not for us to say. We men cannot "count the host of heaven, and call them by their names."
Of God and His Creatures: 2.98