Phantasia can mean several things:
Phantasia is the official transliteration of the Japanese name of an anime. It was eventually given the name The Girl from Phantasia for distribution in North America. The extra words in the title were added to prevent confusion with the Disney animated feature film, Fantasia.
According to Aristotle, phantasia is the process by which all images are presented to us—our desire for the mind to mediate anything not actually present to the senses with a mental image.
Phantasia is the orchestral interpretation of Andrew Lloyd Webber's 1986 musical, The Phantom of the Opera. The interpretation was created by Andrew and Julian Lloyd Webber. 
Phantasia is a Role Playing Game (RPG) included in the book Darksword Adventures (1989) by Margaret Weis & Tracy Hickman.
Phantasia is a rock band from Norborne, Missouri (circa 1969-1973) which included future Rainmakers leader Bob Walkenhorst.
Tales of Phantasia is part of the Tales series