The Collaborative International Dictionary of English v.0.48:

Unsubstantial \Unsubstantial\
   See substantial.

The Collaborative International Dictionary of English v.0.48:

Unsubstantial \Un`sub*stan"tial\, a.
   Lacking in matter or substance; visionary; chimerical.
   [1913 Webster]

WordNet (r) 3.0 (2006):

unsubstantial
    adj 1: lacking material form or substance; unreal; "as
           insubstantial as a dream"; "an insubstantial mirage on
           the horizon" [syn: insubstantial, unsubstantial,
           unreal] [ant: material, real, substantial]

Moby Thesaurus II by Grady Ward, 1.0:

182 Moby Thesaurus words for "unsubstantial":
   Barmecidal, Barmecide, adulterated, airy, apparent, apparitional,
   asomatous, astral, attenuate, attenuated, autistic, bodiless,
   breakable, brittle, cheap-jack, chimeric, cobwebby, crumbling,
   crumbly, cut, dainty, dangerous, decarnate, decarnated, deceptive,
   decrepit, delicate, delicately weak, delusional, delusionary,
   delusive, delusory, dereistic, desultory, dilute, diluted,
   discarnate, disembodied, disintegrating, dreamlike, dreamy,
   effeminate, erroneous, ethereal, extramundane, fallacious, false,
   fanciful, fantastic, feeble, fine, flimsy, fragile, frail,
   frangible, gaseous, ghostly, gimcrack, gimcracky, gossamery,
   hazardous, illusional, illusionary, illusive, illusory, imaginary,
   immaterial, impalpable, imponderable, improbable, inconceivable,
   inconclusive, incorporate, incorporeal, incredible, infirm,
   insecure, insubstantial, intangible, jerry, jerry-built, light,
   lightweight, merely nominal, metaphysical, misleading, namby-pamby,
   nonmaterial, nonphysical, nonsubstantial, not real, occult,
   ostensible, otherworldly, papery, pasteboardy, perilous,
   phantasmagoric, phantasmal, phantom, poor, poorish, precarious,
   provisional, psychic, puny, rare, rarefied, risky, rotten,
   rotten at, seeming, self-deceptive, self-deluding, shadowy, shaky,
   shattery, shifting, shifty, sissified, sleazy, slight, slippery,
   specious, spectral, spiritual, subtile, subtle, supernatural,
   supposititious, tacky, temporary, tentative, tenuous, thick, thin,
   thinned, thinned-out, ticklish, transmundane, treacherous,
   unactual, unbelievable, uncompact, uncompressed, unconcrete,
   unconvincing, undependable, unearthly, unembodied, unextended,
   unfaithworthy, unfirm, unfleshly, unfounded, unphysical, unproved,
   unreal, unrealistic, unreliable, unrigorous, unsolid, unsound,
   unstable, unsteadfast, unsteady, unsturdy, unsubstanced, unsure,
   unsustained, untrustworthy, unworldly, vaporous, visionary,
   watered, watered-down, weak, weakly, weightless, windy, wispy,
   womanish