Something that is unprecedented is not known, experienced, or done before. If you've never gone on a family beach vacation but you're planning one now, you could refer to it as an unprecedented decision.

The root of this word is precedent, a noun referring to something done or said that is used as an example to be followed in the future. In law, a precedent is a legal decision that is used as a standard in future cases. So the adjective unprecedented, meaning "having no precedent," was formed from the prefix un- "not," the noun precedent, and the suffix –ed "having."

Definitions of unprecedented
  1. adjective
    having no precedent; novel
    “an unprecedented expansion in population and industry”
    new, unexampled
    having no previous example or precedent or parallel
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    having or supported or justified by a precedent
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