Concise Oxford English Dictionary © 2008 Oxford University Press:
▶nounthe occurrence and development of events by chance in a happy or beneficial way.
– origin 1754: coined by Horace Walpole, suggested by The Three Princes of Serendip, the title of a fairy tale in which the heroes were always making fortunate discoveries.
I missed the tramp who cycled passed wearing a green plastic colander for a helmet. But I did find this.