Serendipity sounds old, but it is actually a coined word. In 1754, the English writer Horace Walpole used it in a letter. He said he formed it from the title of the Persian fairy tale The Three Princes of Serendip.
In that story, the princes were always making discoveries by accident and through observation. Walpole wanted a word for that kind of happy, unexpected finding. That is why serendipity means more than simple luck. It usually suggests an accidental discovery that also involves noticing something valuable.
For example, you might go to a library looking for one book and come across another that perfectly answers your question. That is serendipity. Scientists and writers also use the word for useful discoveries that were not the original goal.
- Luck: something good happens by chance.
- Serendipity: you unexpectedly find something valuable, often because you recognize its importance.
So the word itself was a deliberate invention, but it describes accidents that turn out wonderfully well.