Never before seen, done, or experienced; without precedent.
"The sudden eruption of a new volcano in that region was an unprecedented event that had never been witnessed by scientists before."
In plain English: Unprecedented means something has never happened before and there is no past example to compare it to.
"The company faced unprecedented challenges during the pandemic that no expert could have predicted before."
Usage: Use unprecedented to describe events that have no prior example in history rather than simply those that are unusual. It is often confused with words like historic, but the key distinction is that something unprecedented has never happened at all before.
The word comes from combining the prefix un- with precedent and the suffix -ed. It originally meant something that had no prior example or model.