Lacking physical substance or solidity; not having a firm or solid form. The unsubstantial mist hung over the valley, giving it an ethereal appearance.
Lacking strength or solidity; flimsy or weak. The unsubstantial argument failed to convince the jury of the defendant's innocence.
Lacking in reality or factual basis; not based on solid evidence. His unsubstantial claims about the new policy were quickly dismissed by the committee.