Impressionism is a term more frequently used with regard to painting and music than to literature and especially with artistic movements occurring in France in the late nineteenth century. A related trend is that of the French Symbolist poets of the same period.
Conrad's prose can be seen in conjunction with these movements because of a deliberate subtlety and even vagueness in his descriptions and his wording overall. In Heart of Darkness, the entire episode of Marlow searching for and finally discovering Kurtz has the quality of a strange dreamlike fantasy. Though the narrative material could not be more different, the tone, at least, of Conrad's story is similar to that of one of the iconic Symbolist poems, Stephan Mallarmé 's "Afternoon of a Faun" (known also through Debussy's musical treatment, a central work of Impressionism in music).
Conrad's story seems covered by a veil, a kind of blur. We are nevertheless first given a sharp description of the dysfunctional nature of colonialism as Marlow arrives at the port, with disused equipment laid about and sick men being given no treatment. The difficulties of travel to the interior are also made clear to the reader, so that the whole expedition comes across as a nightmare.
But the central issue—the madness of Kurtz and his intended demigod-like stance with regard to the "natives"—is never really defined explicitly, never given directly to us in unequivocal wording. Kurtz is an ambiguous figure, depicted by Conrad (at least through Marlow's eyes) as more a victim than victimizer.
Conrad's stylistic approach supports this self-conscious vagueness and ambivalence. The long, intricate sentences and the frequent difficulty one has in following them give the impression of a dream, a blurred series of images in which the ultimate picture of destruction and abuse caused by the colonial system emerges as a puzzle which Marlow will never be able to solve. This blurred and remote theme imparted by the story is one from which we receive "impressions," ones of mystery and confusion, like those of symbolist poetry and impressionism in music, which state their "messages" in the form of dreamlike puzzles as well.