The Impressionist movement in music is loosely set between the late nineteenth century, up to the middle of the twentieth century. Like its precursor in the visual arts, musical impressionism was based in France. Claude Debussy and Maurice Ravel are considered to be the two "great" impressionists (however, Debussy renounced the term). The greatest American impressionist composer is Charles Tomlinson Griffes.
Philosophically, impressionism aimed to convey the emotional impact of an event, place, or thing, rather than an accurate portrayal of the subject itself. For instance, Debussy's setting of the Mallarme poem in Prélude à l'après-midi d'un faune is not a literal portrayal of the events of the already vague poem, but a depiction of the feeling of the poem.
Technically, the impressionists invented or began using a great number of new compositional techniques: multi-modality, planing (the use of voices moving in parallel motion; Debussy's prelude "La cathédrale engloutie" provides an example), extended tertian harmonies, and intentionally ambiguous musical forms. Also, a sharp focus on tone color led to many new orchestrational advances.