serendipity (n.)
Serendipity is finding something valuable without setting out to find it.
In a letter dated January 28, 1754, Horace Walpole described a discovery he had made while looking through an old book at random. He had found the significance of a Venetian coat of arms in a painting. He called this kind of discovery serendipity.1
He remembered The Three Princes of Serendip. As the princes traveled, they found things they had not set out to find, using both chance and their own judgment. Serendip was an old name for Sri Lanka. Walpole took the name from the story.1
Walpole’s word held both parts of the story. The princes came upon things by accident, then worked out what they had found. A lucky find could be missed by someone who did not know what to look at.1
Finding that letter in the archive was pure serendipity.