Source : Webster's Revised Unabridged Dictionary (1913)

Unsubstantial \Un`sub*stan"tial\, a.
   Lacking in matter or substance; visionary; chimerical.

Source : WordNet®

unsubstantial
     adj : lacking material form or substance; unreal; "as
           insubstantial as a dream"; "an insubstantial mirage on
           the horizon" [syn: {insubstantial}, {unreal}] [ant: {substantial}]