Objection 1. It would seem that not everything which God does outside the natural order of things, is miraculous. For the creation of the world, and of souls, and the justification of the unrighteous, are done by God outside the natural order; as not being accomplished by the action of any natural cause. Yet these things are not called miracles. Therefore not everything that God does outside the natural order is a miracle.
Objection 2. Further, a miracle is "something difficult, which seldom occurs, surpassing the faculty of nature, and going so far beyond our hopes as to compel our astonishment" [St. Augustine, De utilitate credendi xvi.]. But some things outside the order of nature are not arduous; for they occur in small things, such as the recovery and healing of the sick. Nor are they of rare occurrence, since they happen frequently; as when the sick were placed in the streets, to be healed by the shadow of Peter (Acts 5:15). Nor do they surpass the faculty of nature; as when people are cured of a fever. Nor are they beyond our hopes, since we all hope for the resurrection of the dead, which nevertheless will be outside the course of nature. Therefore not all things are outside the course of natur are miraculous.
Objection 3. Further, the word miracle is derived from admiration. Now admiration concerns things manifest to the senses. But sometimes things happen outside the order of nature, which are not manifest to the senses; as when the Apostles were endowed with knowledge without studying or being taught. Therefore not everything that occurs outside the order of nature is miraculous.
On the contrary, Augustine says (Contra Faust. xxvi, 3): "Where God does anything against that order of nature which we know and are accustomed to observe, we call it a miracle."
I answer that, The word miracle is derived from admiration, which arises when an effect is manifest, whereas its cause is hidden; as when a man sees an eclipse without knowing its cause, as the Philosopher says in the beginning of his Metaphysics. Now the cause of a manifest effect may be known to one, but unknown to others. Wherefore a thing is wonderful to one man, and not at all to others: as an eclipse is to a rustic, but not to an astronomer. Now a miracle is so called as being full of wonder; as having a cause absolutely hidden from all: and this cause is God. Wherefore those things which God does outside those causes which we know, are called miracles.
Reply to Objection 1. Creation, and the justification of the unrighteous, though done by God alone, are not, properly speaking, miracles, because they are not of a nature to proceed from any other cause; so they do not occur outside the order of nature, since they do not belong to that order.
Reply to Objection 2. An arduous thing is called a miracle, not on account of the excellence of the thing wherein it is done, but because it surpasses the faculty of nature: likewise a thing is called unusual, not because it does not often happen, but because it is outside the usual natural course of things. Furthermore, a thing is said to be above the faculty of nature, not only by reason of the substance of the thing done, but also on account of the manner and order in which it is done. Again, a miracle is said to go beyond the hope "of nature," not above the hope "of grace," which hope comes from faith, whereby we believe in the future resurrection.
Reply to Objection 3. The knowledge of the Apostles, although not manifest in itself, yet was made manifest in its effect, from which it was shown to be wonderful.
Source: Summa Theologica (New Advent)