Definition of phantasia:
(n) :
(dated) Something imaginary; a fantasy.
(n) :
(philosophy) A phantasm (an impression received through the senses) or the faculty of receiving or representing these impressions.
(n) :
Archaic spelling of fantasia. [(music, also figurative) A form of instrumental composition with a free structure and improvisational characteristics; specifically, one combining a number of well-known musical pieces.]