As I understand it the UCI didn't ban recumbents, merely introduced a set of regulations which made it impossible for a 'bent to comply with. It has the same effect but doesn't see them being accused of outlawing a particular type of bike from competition. Same as with Graeme Obree's hour record bike, the UCI declared that the riders chest couldn't rest on the handlebars, which his bike sort of relied on.
Answering the original question, a quick Google turned this up;
Bicycle
/ˈbʌɪsɪkl/
Noun
A vehicle consisting of two wheels held in a frame one behind the other,
propelled by pedals and
steered with
handlebars attached to the front wheel.
So a two-wheel 'bent can be defined thus.