Debussy's music adapted to the functions of background music because of its fluidity and non-assertiveness. It's unobtrusive. So Art Nouveau and related literary movements may provide a better analogy than "Impressionism."
Alongside these masterpieces of orchestral evocation, Debussy also developed a highly personal piano style. Perhaps the first piece to show a marked difference from his earlier piano music was the first piece from the two books of Images for piano, ‘Reflets dans l’eau’. Its rippling arpeggios, bathed in pedal, pioneered a new Impressionist piano style.
Most notable among these later pieces is a work that is the closest thing to a symphony that Debussy wrote, the symphonic suite La Mer. Comprised of three symphonic movements Debussy called "sketches," the work is a musical "impression" (for lack of a better term, and one that Debussy loathed) of the sights and sounds of the ocean.
In reaction to Wagner and his highly elaborate late-romantic operas, Debussy wrote the symbolist opera Pelléas et Mélisande, which would be his only finished opera. Based on the play by Maurice Maeterlinck, the opera proved to be immensely influential to younger French composers, including Maurice Ravel. Pelléas, with its rule of understatement and deceptively simple declamation, also brought an entirely new tone to opera — but an unrepeatable one.
It was not until 1894, aged 32, that Debussy completed the first piece to truly declare his independence of thought: Prelude a l'Apres-midi d'un Faune, a highly innovative piece inspired by a poem of Stephane Mallarme. Spurred on by its success, Debussy began serious work on his opera Pelleas et Melisande and the three orchestral Nocturnes.
The success of Pelleas et Melisande's long-delayed premiere in 1902 made Debussy a celebrity.
Unlike many composers, Debussy seems not to have spent his early teens covering sheet after sheet of manuscript paper with symphonies, concertos and aborted five-act operas. No work of his can be definitely dated before he song 'Madrid' dating from the spring of 1879, when he was sixteen and a half, while his first extended work, the Piano Trio in G, was written during September and October 1880.
After some 300 years of pulsating Germanic music, for Debussy to come along and write such hauntingly restrained, ethereal, time-stands-still works was a shock to the system. His thick yet transparent block chords; his harmonies tinged with ancient modal elements; his preference for whole-tone scales that loosened music's moorings to traditional tonality; his mastery of delicate orchestral colorings and new ways of writing for the piano: all this and more made him the father of modern music. Composers from Stravinsky to Boulez would have been impossible without Debussy's example.
Issuing from a background that did not mold his taste, but rather challenged him to define one, Debussy resist convention from the start, seeking alternatives to the language he was taught at the Conservatoire. Belonging to no one group or class, but crossing many, […], Debussy abjured "the orthodox," retaining his independence and ability to both assimilate and transform his models.
So was Debussy an Impressionist? In his authoritative entry on Debussy for the 2001 edition of The New Grove Dictionary of Music and Musicians, the French musicologist François Lesure strongly argues no. He places Debussy in the Symbolist movement in French literature and arts, which thrived for about a dozen years starting in 1885. It’s an aesthetic Debussy characterized, Lesure writes, by “rejection of naturalism, of realism and of overly clear-cut forms, hatred of emphasis, indifference to the public and a taste for the indefinite, the mysterious, even the esoteric.”
Debussy is often categorized as an “Impressionist” composer. This designation, which was first applied to painters of the same era, refers to the evoking of a mood using harmony and tone color. It is this style of composition with which Debussy is most closely associated, despite the fact that he tended to disavow the label.