An unsubstantiated claim is one that nobody has backed up with evidence. Notice carefully what this does and does not say. It does not say the claim is false. It says no proof has been shown — the claim is just sitting there, unsupported. That gap is exactly why the word exists, and why careful people reach for it. Journalists and lawyers use it constantly, because it lets them report that someone said something serious without agreeing that it is true: 'unsubstantiated allegations', 'unsubstantiated rumours', 'the report remains unsubstantiated'. It is a formal, written word — news, court, official inquiries, company statements. In conversation you would say 'there's no proof' instead. It nearly always sits in front of a small set of nouns: claim, allegation, rumour, report, accusation, complaint. Those nouns tell you the flavour: unsubstantiated usually attaches to something accusing, something that could damage a person if believed. There is a matching verb, 'substantiate' (to prove with evidence), which you will meet in the phrase 'substantiate a claim'.