A three-wheeled vehicle, most commonly a pedal cycle. Developed as a more staid and gentlemanly form of locomotion than the bicycle in the late 19th century, but mostly died out as a form of adult transport on the entirely reasonable grounds that they offer negligible advantages over their two-wheeled counterparts in return for considerable increases in mechanical complexity, rolling and wind resistance, storage space and road room, and because they are actually rather harder to ride and less stable than a bicycle at any speed over about 4 mph.
Since then, upright tricycles have been a refuge of the determinedly unconventional (hey, even bicyclists are still considered as enough of a joke in most English-speaking countries) and people with inner ear problems. In the UK, there is still a Tricycle Association and even a small number of races, and since the original date of this writeup Belgium seems to have taken trike racing up in a small way as well, enough to hold an unofficial annual World Championships; I do not know whether my own occasional outings on a barrow when I lived there had any influence on this. The leading manufacturer of lightweight machines is George Longstaff while Pashley make utility models; a notable but defunct maker was the Higgins company. Most makers also produced "conversion kits" to make a trike from a bike frame. Recumbent models are a more recent development and somebody else can node about them; they're easy, practical and sensible and much less entertaining.
How to ride a tricycle
You might assume that an upright tricycle is easy to ride, since it stands up on its own. If you have never ridden a bicycle, this is possibly the case; however, if you have, then you have a shock coming. This machine will actively attempt to throw you off at any opportunity it sees. It will take control of its own steering, usually in the direction of the kerb (hey, don't complain, at least that's away from the oncoming traffic). This is a serious challenge.
All you have to fear, however, is your own reactions, especially your own panic reactions. On a bicycle, you steer into a wobble, in order to get the wheels under your centre of gravity; on a trike you have to pull away from it, so experienced bike riders mostly just make the problem worse and end up going in ever decreasing circles until they fall off. Indeed, you do most of the steering on a bike with your arse, by shifting your weight from one side to another; on a trike, you have to knowingly turn the handlebars and then shift your weight in such a way as to stop the thing turning turtle; in a hard turn this normally involves hanging out with your left buttock over the right hand wheel, or vice versa; to facilitate this you put your inside foot down whilst cornering (whereas on a bike you raise it to increase ground clearance while you lean the machine over. You're trying not to do that, though, remember?).
Your next obstacle is the roadway itself (no, don't even think about going offroad). The surface may look flat to you now, but you will soon learn that highway engineers have some odd ideas about cambering it so that the rainwater runs off into the gutters. The trike will tend towards the fall line, so you normally have to keep hauling on the bars to hold a straight line; indeed, if the camber is hard enough, you may end up riding with both hands pulling on the uphill side of the bars. Small irregularities like road mends and sunken drain covers will send you veering wildly in a different direction; with luck there won't be any major obstacles there.
Once you have mastered this (which requires constant attention for the first couple of hours, lest you forget and revert to two-wheeled ways), you will be able to give up on the wholly reasonable grounds (have I said that before) that there is no reason to go to all this additional effort just in order to have everybody go into hysterics whenever they catch sight of you, and go back to a nice, sensible, man-and-machine-in-perfect-harmony, bicycle. You will then, however, probably forget to take your feet off the pedals when you stop.