Unrealistic or impractical.
Of no immediate practical application.
Fictitious or nonexistent.
Illusive.
A ghost or apparition.
A placeholder for a pair of players when there are an odd number of pairs playing.
Something apparently seen, heard, or sensed, but having no physical reality; an image that appears only in the mind; an illusion or delusion.
A test object. A test phantom is an object that reproduces the characteristics of human tissue.