Miracle(s) is the topic of the book by that name. Lewis defines Miracle in Miracles 2.1
Miracles 2.1 Miracle means an interference with Nature by supernatural power.
Miracles 6.7 "The presence of human rationality in the world is therefore a Miracle by the definition given in Chapter II." This then is the First Miracle Lewis demonstrates and continues: "On realizing this the reader may excusably say:'Oh if that's all he means by a Miracle ... ' and fling the book away. But I ask him to have patience. ..."
Miracles 6.8 "Does He [God] , besides all this, ever introduce into her events of which it would not be true to say, 'This is simply the working out of the general character which He gave to Nature as a whole in creating her'? Such events are what are popularly called Miracles: and it will be in this sense only that the word Miracle will be used for the rest of the book."

I use the word Miracle to mean an interference with Nature by supernatural power.

Footnote:
This definition is not that which would be given by many theologians. I am adopting it not because I think it an improvement upon theirs but precisely because, being crude and "popular," it enables me most easily to treat those questions which "the common reader" probably has in mind when he takes up a book on Miracles. Miracles, chapter 2, paragraph 1

The question whether miracles occur can never be answered simply by experience. Every event which might claim to be a miracle is, in the last resort, something presented to our senses, something seen, heard, touched, smelled, or tasted. And our senses are not infallible.… What we learn from experience depends on the kind of philosophy we bring to experience. Miracles, chapter 1, paragraph 2

Belief in miracles, far from depending on an ignorance of the laws of nature, is only possible in so far as those laws are known. We have already seen that if you begin by ruling out the supernatural you will perceive no miracles. We must now add that you will equally perceive no miracles until you believe that nature works according to regular laws. If you have not yet noticed that the sun always rises in the East you will see nothing miraculous about his rising one morning in the West. Miracles, chapter 7, paragraph 3

If the laws of Nature are necessary truths, no miracle can break them: but then no miracle needs to break them. It is with them as with the laws of arithmetic. If I put six pennies into a drawer on Monday and six more on Tuesday, the laws decree that other things being equal--I shall find twelve pennies there on Wednesday. But if the drawer has been robbed I may in fact find only two. Something will have been broken (the lock of the drawer or the laws of England) but the laws of arithmetic will not have been broken. The new situation created by the thief will illustrate the laws of arithmetic just as well as the original situation. But if God comes to work miracles, He comes "like a thief in the night." Miracle is, from the point of view of the scientists, a form of doctoring, tampering, (if you like) cheating. It introduces a new factor into the situation, namely supernatural force, which the scientists had not reckoned on. Miracles, chapter 8, paragraph 8

All the essentials of Hinduism would, I think, remain unimpaired if you subtracted the miraculous, and the same is almost true of Mohammedanism. But you cannot do that with Christianity. It is precisely the story of a great Miracle. A naturalistic Christianity leaves out all that is specifically Christian. Miracles, chapter 10, paragraph 1

Christianity, faced with popular "religion" is continuously troublesome. To the large well-meant statements of "religion" it finds itself forced to reply again and again, "Well, not quite like that," or, "I should hardly put it that way." This troublesomeness does not of course prove it to be true; but if it were true it would be bound to have this troublesomeness. The real musician is similarly troublesome to a man who wishes to indulge in untaught "musical appreciation"; the real historian is similarly a nuisance when we want to romance about "the old days" or "the ancient Greeks and Romans." The ascertained nature of any real thing is always at first a nuisance to our natural fantasies--a wretched, pedantic, logic-chopping intruder upon a conversation which was getting on famously without it. Miracles, chapter 11, paragraph 8

[In the miracles that fall in the class of Miracles of Fertility] are the two instances of miraculous feeding. They involve the multiplication of a little bread and a little fish into much bread and much fish. Once in the desert Satan had tempted Him to make bread of stones: He refused the suggestion. "The Son does nothing except what He sees the Father do"; perhaps one may without boldness surmise that the direct change from stone to bread appeared to the Son to be not quite in the hereditary style. Little bread into much bread is quite a different matter. Every year God makes a little corn into much corn: the seed is sown and there is an increase. Miracles, chapter 15, paragraph 8

The Miracles of Reversal all belong to the New Creation. It is a Miracle of Reversal when the dead are raised. Old Nature knows nothing of this process: it involves playing backward a film that we have always seen played forwards. The one or two instances of it in the Gospels are early flowers; what we call spring flowers, because they are prophetic; although they really bloom while it is still winter. Miracles, chapter 15, 2nd paragraph from the end.

"You are quite right. The laws of the universe are never broken. Your mistake is to think that the little regularities we have observed on one planet for a few hundred years are the real unbreakable laws; whereas they are only the remote results which the true laws bring about more often than not; as a kind of accident."

"Shakespeare never breaks the real laws of poetry," put in Dimble. "But by following them he breaks every now and then the little regularities which critics mistake for the real laws. Then the little critics call it a 'licence.' But there's nothing licentious about it to Shakespeare."

"And that," said Denniston, "is why nothing in Nature is quite regular. There are always exceptions. A good average uniformity, but not complete." That Hideous Strength, chapter 4, paragraphs 10-13

The miracle has only half its effect if it only convinces us that Christ is God: it will have its full effect if whenever we see a vineyard or drink a glass of wine we remember that here works He who sat at the wedding party in Cana. Miracles—a sermon, paragraph 9

This would fall in with an old opinion of my own that we ought all of us to be ashamed of not performing miracles and that we do not feel this shame enough. We regard our own state as normal and theurgy as exceptional, whereas we ought perhaps to regard the worker of miracles, however rare, as the true Christian norm and ourselves as spiritual cripples. Petitionary Prayer, last paragraph