Fairy comes from the Old French word fae. The word has been overused to describe a supernatural being. However, there is a great deal of difference in classifying a being as a fairy from the medieval literature and those from modern literature, especially those belonging to the Celtic tradition.
The Flower fairies of the Victorian age are tiny and harmess creatures whereas the trolls of Scandinavia could blow an entire village. Are we talking about the same kind of creatures ? Do they share common traits and attributes ?