Serendipity is that beauty of the internet that allows you to click on a single thread and an hour later find yourself a million clicks away. For a definition you can click here: https://en.wiktionary.org/wiki/serendipity
Johnson uses it to mean those unexpected, unsought, or unintended collisions and experiences that we happen into that we learn, grow, or gain inspiration from. We all sort of know what this is... going to the office supply store and running into a business contact, or working for an hour in a meeting to figure out you're missing a piece to the puzzle and nobody knows what it is. But you get back to your office to find a note that while you were out, Mr. Ash from the Weyland-Yutani Corporation stopped by and has the missing 'thing' you need. Yeah, that kind of thing is Serendipity.
Co-Working spaces, coffee shops, and bars are great places for these kind of collisions to occur. There are lots of ideas colliding in chapter four, also. The random nature of serendipity brings on the obvious question, 'how do we plan for the unplanned, the unexpected, and the unintended?' Well fortunately, some of the discussion is geared to answering just that line of questioning. In Tony Hsieh's video, he talks about having an apartment that overlooked the space his company wanted to change. Elon Musk has an office over the shop floor at Tesla. The old school barber shop and places like that where people can meet and talk business or just talk are designed for serendipity. Google has a portion of the week set aside for engineers to work on whatever interests them. So there are ways of bringing this about. Part of designing with serendipity in mind is simply being aware that it needs to happen. Set aside that time on your calendar that is reserved for doing nothing, then go out and do something ... like watch the video below to hear Stephen Johnson say it for himself on the TED stage.