I said the sense "stories about fairies" was too narrow. It is too narrow, even if we reject the diminutive size, for fairy-stories are not in normal English usage stories about fairies or elves, but stories about Fairy, that is Faërie, the realm or state in which fairies have their being. Faerie contains many things besides elves and fays, and besides dwarfs, witches, trolls, giants, or dragons: it holds the seas, the sun, the moon, the sky; and the earth, and all things that are in it: tree and bird, water and stone, wine and bread, and ourselves, mortal men, when we are enchanted.
The definition of a fairy-story -- what it is, or what it should be -- does not, then, depend on any definition or historical account of elf or fairy, but upon the nature of Faërie: the Perilous Realm itself, and the air that blows in that country. I will not attempt to define that, nor to describe it directly. It cannot be done. Faërie cannot be caught in a net of words; for it is one of its qualities to be indescribable, though not imperceptible. It has many ingredients, but analysis will not necessarily discover the secret of the whole. Yet I hope that what I have later to say about the other questions will give some glimpses of my own imperfect vision of it. For the moment I will say only this: a "fairy-story" is one which touches on or uses Faërie, whatever its own main purpose may be: satire, adventure, morality, fantasy. Faërie itself may perhaps most nearly be translated by Magic - but it is magic of a peculiar mood and power, at the furthest pole from the vulgar devices of the laborious, scientific, magician. There is one proviso: if there is any satire present in the tale, one thing must not be made fun of, the magic itself. That must in that story be taken seriously, neither laughed at nor explained away.
by J. R. R. Tolkien in "On Fairy Stories" in Tree and Leaf