Serendipity is a chance observation or accident that leads, through a real subsequent chain of reasoning, skill, and effort, to a valuable discovery. The word distinguishes this from a lucky guess: the chance event supplies an opportunity, but recognizing its significance and following through still requires the observer’s expertise and work — as this site’s own case study of Fleming’s discovery of penicillin shows in detail.
Serendipity is one of this site’s taxonomy categories precisely because it’s a coincidence type with a clear, checkable causal chain running from the chance event to the discovery — unlike categories where the analytical question is whether any connection exists at all.