1 Broad street, promenade, or walk, planted with rows of trees, e.g. those laid out by Haussmann in C19 Paris.
2 Horizontal portion of a rampart, hence the promenade laid out on demolished fortifications, e.g. the Ring streets around many German cities. One of the first boulevards (1670) was created on the line of walls flattened by Vauban in Paris, running from the Porte St-Denis to the Bastille, intended as a promenade for pedestrians only.
3 Any wide main street, especially in the USA.
Goulty (1991);Symes (1993)