A Ferris Wheel is an amusement park ride that consists of multiple seats on a large, rotating wheel. It is often used by couples for romantic moments in the real world.
...you're still here. All right, fine, you obviously want the Ponychan definition of Ferris Wheel.
In the context of Ponychan, a ferris wheel is a user shipping roleplay that "takes place" on a ferris wheel. Ferris Wheels were invented and primarily done by a user named Rarity Sauce, who has since changed names but keeps !Sauce in their tripcode. The first ever ferris wheel was done between Rarity Sauce and Lucky Streak (who is now much more inactive, and appears only in /rp /) in a thread started by HawkeyePierce on /oat /, with Amonisis as the Ferris Wheel itself. A certain Gyropedian knows he has that damn thread saved somewhere, but he can't find it. He also had a MegaUpload link lying around, but he's dumb so he lost it. It's a good thing a certain prince still had the thread.
All you need to know for now is that the first one involved Rarity Sauce getting really kinky and whipping Lucky Streak on the ferris wheel, much to the amusement of park-goers. Most ferris wheels, however, don't involve BDSM, probably per the mods' request. They eventually cracked down completely, not allowing new Ferris Wheel threads to be created, and cracking down on those that are made. Ferris Wheel threads are currently allowed as long as they don't devolve into clopfic territory. Ferris wheels have stirred up trouble more than once, and much like syrup, remain one of the most controversial /oat/ memes to date.