Serendipity is an unplanned fortunate discovery. It is the faculty or phenomenon of finding valuable or agreeable things not sought for. The term "serendipity" was first used in the mid-1700s by English author Horace Walpole, who coined it as an allusion to Serendip, an old name for Sri Lanka. He was inspired by the fairy tale of The Three Princes of Serendip, in which the three brothers stumbled upon happy discoveries without looking for them.

Serendipity is often applied to inventions made by chance rather than intent, and it is a design principle for online activity that would present viewpoints that diverge from those participants already hold. Harvard Law professor Cass Sunstein argues that such an "architecture of serendipity" would promote a healthier democracy.

Serendipity can bring simple pleasures and happiness to everyday life, and the surprise factor associated with serendipity has the power to trigger our minds, prompt us to think, and learn more. Acknowledging or recognizing a moment as potentially significant to oneself personally is what sets serendipity apart from luck.