Fantastic is a literary term that describes a quality of other literary genres, and in some cases is used as a genre in and of itself, although in this case it is often conflated with the Supernatural. The term was originated in the structuralist theory of critic Tzvatan Todorov in his work The Fantastic. He describes the fantastic as being a liminal state of the supernatural.
A truly fantastic work is subtle and leaves the reader with a sense of confusion about the work about whether or not the phenomenon was real. Todorov compares this with two other ideas: The Uncanny, wherein the phenomenon turns out to have a rational explanation such as in the Gothic works of Ann Radcliffe; or the Marvellous, where there truly is a supernatural explanation for the phenomenon.