A bus that is fitted with special steering equipment to allow it to be driven along a narrow busway without the driver needing to steer. As such it combines some of the benefits of light rail with the flexibility of a bus that can be manually steered on normal roads.
A good example of a long-distance guided busway can be found near Cambridge. The construction project was controversial, as many considered that light rail would have been a better option. However it is popular with people cycling due to a parallel cycleway (a service road) being included in the design.