Something which is unsubstantiated is not supported by facts or not proven to be true. Something described as being 'unsubstantiated' might just be a rumour or a claim made without any evidence to back it up. Unsubstantiated is similar in meaning to 'unconfirmed' 'uncorroborated' or 'unsupported'. This is an adjective used often in academic and legal contexts.

1. The politician stated that the claims made by the journalist were unsubstantiated.
2. The newspaper apologised for writing about the unsubstantiated allegations made against the pop-star.

1. The documentary made many claims about health but they were generally found to be unsubstantiated by science or hard facts.
2. Consumers generally avoid products that make claims which are unsubstantiated by clinical data.