It´s the name of one of Barcelona's broadest and most important avenues. It cuts the city in two, diagonally from west to east, hence the name.
It was originally projected by engineer and urban planner Ildefons Cerdà as one of the city's wide avenues, which along with Avinguda Meridiana would cut the rationalist grid he designed for l'Eixample (Catalan for extension). Both would meet at Plaça de les Glòries Catalanes, which Cerdà envisioned as the new city centre. However, Plaça Catalunya, equally a new addition to the city of Barcelona, and connecting Ciutat Vella and Eixample, and therefore occupying a more privileged position in the urban area, would finally become the centre. Avinguda Diagonal remains to this day a much-transited avenue and many companies and hotels use it as a privileged location, as can be seen in its architecture.