Never having happened before — or never having happened on this scale. A precedent is an earlier case that shows this kind of thing has happened, so if something is unprecedented, there is no earlier case like it to point back to. Two slightly different uses hide inside that one word. Sometimes it means the event itself is brand new in kind: an unprecedented decision, a country doing something no country has done before. Sometimes the event is ordinary but the size or number is new: unprecedented rain, unprecedented demand, unprecedented numbers of visitors. It rains every year, so the second use is really "more than ever recorded". The word is formal. You meet it constantly in news reports, business talk, and government statements, and almost never between friends — a friend says "I've never seen anything like it". It is also, honestly, a word people stretch. Journalists and advertisers reach for it because it sounds dramatic, and much of what they call unprecedented turns out to have happened in 1987 or 1953; strictly it should mean nobody has a record of this before, but in practice it often just means "very unusual, very big". It leans negative — crises, disasters, price rises — though it is not required to; unprecedented growth and unprecedented success are perfectly normal. There is no comparison: something either has a precedent or it does not, so careful writers avoid "more unprecedented", even though you will hear it.