"unsubstantial" in the adjective sense
1. insubstantial, unsubstantial, unreal
lacking material form or substance unreal
"as insubstantial as a dream"
"an insubstantial mirage on the horizon"
Source: WordNet® (An amazing lexical database of English)
Princeton University "About WordNet®."
WordNet®. Princeton University. 2010.
View WordNet® License