To show with evidence that a claim is true. Someone says something — an accusation, a report, a number in an advertisement — and you substantiate it by producing the documents, the witnesses, the test results that back it up. A formal verb: it belongs to courts, journalism, science and business, and in ordinary talk you would say 'prove' or 'back up' instead. The pattern matters. You substantiate a claim, an allegation, a story, a figure — never a person and never a thing. You cannot substantiate your boss; you substantiate what your boss said. Very often you meet it in the negative, and that is worth knowing: 'unsubstantiated rumours', 'the allegations could not be substantiated'. That phrasing is careful rather than final — it says no one managed to prove the thing, not that the thing is false, and journalists and lawyers choose it precisely because it stops short of saying so.