St. Thomas Aquinas on Miracles

THESE works that are sometimes done by God outside the usual order assigned to things are wont to be called miracles: because we are astonished (admiramur) at a thing when we see an effect without knowing the cause. And since at times one and the same cause is known to some and unknown to others, it happens that of several who see an effect, some are astonished and some not: thus an astronomer is not astonished when he sees an eclipse of the sun, for he knows the cause; whereas one who is ignorant of this science must needs wonder, since he knows not the cause. Wherefore it is wonderful to the latter but not to the former. Accordingly a thing is wonderful simply, when its cause is hidden simply: and this is what we mean by a miracle: something, to wit, that is wonderful in itself and not only in respect of this person or that. Now God is the cause which is hidden to every man simply: for we have proved above[1] that in this state of life no man can comprehend Him by his intellect. Therefore properly speaking miracles are works done by God outside the order usually observed in things.
Of these miracles there are various degrees and orders. The highest degree in miracles comprises those works wherein something is done by God, that nature can never do: for instance, that two bodies occupy the same place, that the sun recede or stand still, that the sea be divided and make way to passers by. Among these there is a certain order: for the greater the work done by God, and the further it is removed from the capability of nature, the greater the miracle: thus it is a greater miracle that the sun recede, than that the waters be divided. The second degree in miracles belongs to those whereby God does something that nature can do, but not in the same order: thus it is a work of nature that an animal live, see and walk: but that an animal live after being dead, see after being blind, walk after being lame, this nature cannot do, but God does these things sometimes by a miracle. Among these miracles also, there are degrees, according as the thing done is further removed from the faculty of nature.
The third degree of miracles is when God does what is wont to be done by the operation of nature, but without the operation of the natural principles: for instance when by the power of God a man is cured of a fever that nature is able to cure; or when it rains without the operation of the principles of nature.

Summa Contra Gentiles 3:101