A design of vehicle body where the entire structure is made of pieces of sheet metal, bent, folded, and what have you, instead of pieces of sheet metal bolted to a chassis.
The advantage of unibody design is that it is lighter (a chassis tends to be heavy) and absorbs the shock of an accident better. Some argue that these vehicles are not as sturdy as vehicles built on a chassis - some are and some aren't. Unibody vehicles are designed so that the entire vehicle absorbs the force of an accident, thus they are more difficult to repair afterwards, but at the same time, they are safer.
Many popular vehicles today are unibodies, including the Mazda Miata, the Dodge Caravan, and the Jeep Cherokee, among others. The biggest disadvantage of a unibody design is in places where salt is used on the roads - you cannot just replace many of the panels, when they rust out, and must instead patch them.