The French call it a 'monocycle' and the cognitively-impaired call it a 'one-wheeled bicycle'.

Whatever you call it, the unicycle has the absolute minimum number of wheels and therefore somewhat more difficult to master than a traditional bicycle. However, it does have a few advantages over other forms of transportation. You can easily take it on a bus or train, and since it generally lacks such luxuries as brakes, gears, or a chain, it has less to go wrong with it. Having no handlebars enables the rider to occupy their hands with some other exciting activity such as juggling or holding splitting sides as someone makes the inevitable 'You're missing a wheel' quip.

The taller variety, typically six to seven feet high, is generally known as a giraffe due to its appearance. Also, for the more enthusiastic rider, there is the mountain unicycle. As if mountain biking was not hard enough already...