Perhaps the best definition that we’ve heard for this is that “Serendipity is the art of finding what you’re not looking for.”
This agrees pretty well with the explanation of Horace Walpole, who coined the word in 1754 after reading the old fairytale The Three Princes of Serendip.
Walpole wrote that these princes “were always making discoveries, by accidents and sagacity, of things they were not in quest of.”
Even though Walpole, in coining the word, made quite clear the meaning he intended it to have, Bernard E. Schaar has conducted research into the matter that convinces him the princes were being maligned (New York World Telegram & Sun, Sept. 10, 1957).
He is convinced, after close study of the fairytale, that the princes of Serendip (now known as Ceylon) were highly educated, and that their seemingly fortuitous discoveries were no more accidental than are the wondrous results achieved by today’s highly trained scientific investigators, whose very training leads them to recognize the worth of an experimental result even when the experiment does not proceed according to plan.