Fantasy refers to the imagining to things that are impossible or improbable.
Fantasy essentially refers to the creative imagination. Anything that you can think of that is not possible or is unlikely to occur is considered as fantasy. Hence, literature that contains magic and supernatural phenomena is often termed as fantasy literature. This type of literature is considered as a genre of fiction. Fantasy books often take in imaginary worlds, where magic and magical creatures are common. Some popular examples of fantasy literature are the Harry Potter series by J.K. Rowling, or the Lord of the Rings by J.R.R. Tolkien.
There is also a genre called as Fantasy Films, which also incorporate similar themes of magic and supernatural occurrences. A number of these movies are often adaptations of fantasy literature.
The term can also be used to refer to the product of the imagination, such as a daydream. Here, a fantasy refers to a situation imagined by an individual that expresses certain desires or aims of its creator. These fantasies often include situations that are highly unlikely or unrealistic. Some fantasies may even be sexual in nature.
The term fantasy is also often relates to sports, such as Fantasy Football, where people collect and manage their own imaginary football team.