Having nothing else beside it that can match it. If you say a singer's voice is unparalleled, you are saying that in all your looking, you cannot find a second one at the same level. It is one of English's strongest words for 'the greatest' — stronger than 'excellent', close to 'the best there has ever been'. Because it is so strong, it lives mostly in formal writing: advertisements, reviews, speeches, history books ('a period of unparalleled growth'). Two things are worth noticing. First, it is not only for good things. 'Unparalleled destruction', 'unparalleled cruelty', 'a disaster of unparalleled scale' — the word means 'nothing matches it', and something can fail to be matched at the bottom just as easily as at the top. Most learners only meet the positive use and are surprised by the other one. Second, it makes a big claim, so it is easy to overuse. If a writer calls three different restaurants unparalleled, the word stops working. The picture inside it helps: parallel lines run beside each other forever, always the same. Something unparalleled has no line beside it at all — it stands alone.