A prolonged visit to wikipedia during which contextual links are followed until the user is reading pages that have nothing to do with the original subject they sought. Particularly exciting wikifaris end up coming back to the original page at some point.
OMG! I went on the most incredible wikifari last night! I started with Nirvana, got all the way to osmotic pressure, and then, 5 hours later, I was at Nirvana again!
Man, there's a test coming up, I better go on a wikifari.