Fantastic: There are several definitions of the word fantastic, you will find it more official and complete in a dictionary.
For my part, I would say that these are unexpected and unexplained events (strange, magical, supernatural etc.) taking place in a story. These strange events can sometimes be explained, but not always. Very often they appear singular and disturbing.
The fantastic questions its author as well as the public. Thus putting us face to face with a universal problem (our greatest fears for example) or specific (an element of society).
The characters will face them to defeat them or not.