Note: Do not rely on this information. It is very old.
Boulevard (a French corruption of the German bollwerk, English bulwark), properly, the rampart of a fortified city. Part of the fortifications of Paris was removed in 1786, and the space converted into avenues flanked by two rows of trees. To these the term was applied, and it was afterwards extended to the similar streets (Boulevard des Capucines, des Italiens, etc.), formed under the Second Empire by Baron Haussmann, which are one of the most striking features of modern Paris. These of course are not on the sites of fortifications. They have been imitated in other French towns, in New York, and, to some extent, in London.