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The previous answer is right in that it refers to a lane of traffic, not the bus itself (as far as I know, anyway).
Here are some refs.
Ampliación y mejoras del carril Bus-Vao de la nacional VI, tramo Moncloa/Las
Rozas- (MADRID), El Sistema de Control y Explotación ...
ORDEN MINISTERIAL de 10/5/1995 POR LA QUE SE DEFINEN LOS CONCEPTOS DE «VEHICULO CON ALTA OCUPACION (VAO)» y «CARRIL RESERVADO PARA LA CIRCULACION DE VEHICULOS CON ALTA OCUPACION» Y SE DETERMINAN LAS CONDICIONES DE UTILIZACION DE ESTE ULTIMO. (BOE: 19/5/1995) http://www.carreteros.org/circulacion/otros/otros.htm
The temporary Bus/High Occupancy Vehicle (Bus/HOV) lanes on I-25 are offered exclusively to buses and motorists who participate in carpooling efforts by traveling with two or more persons in a vehicle and operate during peak drive times. http://www.ridearrangers.org/newhov.html
Explanation: Where HOV stands for Highly Occupied Vehicles (it will need a translator's note). In Madrid, it's called Carril Bus-VAO and it's supposed to be a fast way to reach the city from the north (as an alternative to the lanes of the A-6, the Madrid-La Coruña highway) if you travel by bus or in a vehicle with three or more people.