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.

Impressionism was originally an artistic rather than a literary movement. It was based on the nature of perception, specifically the knowledge that we do not actually see objects, but rather see light falling on objects that is reflected to our eyes. Thus impressionists focused on portraying the light that impresses itself on our eyes rather than reconstructing the object per se. The symbolist poets sometimes were described as impressionists because, in opposition to the realist movement, they believed that we do not experience external events directly but instead as mediated through our senses and sensibilities. Thus poets such as Rimbaud were concerned with the nature of sensation, of trying to describe emotional impressions, rather than to create vivid approximations of an external world. 

Perhaps the most prototypically impressionist novelist, in the strict sense of the term, was Huysman, whose A Rebours focuses on an aesthete concerned with this specific problem of maximizing certain types of sensation in his life. While certain recent genre theorists have discussed Heart of Darkness as an impressionistic novel, stylistically it is far closer to German Expressionist work than to the refined urban sensibilities of the French fin de siecle poets and their imitators (such as Arthur Symons, whose poetry and criticism were seminal in the use of "impressionism" as a literary term). In some ways, Heart of Darkness has almost Gothic characteristics in its exotic locale and atmosphere of horror. Simply expressing the interior thoughts of characters does not make a writer an impressionist, as that is a feature common to almost all novelists. 

Ian Watts's position that Conrad was an impressionistic writer uses the term somewhat ahistorically, not referencing Impressionism within its literary or artistic context, but rather referring to Hume's philosophical concept of impressions and labeling works focused on the interior states of characters, such as those of Conrad and Virginia Wolfe, as impressionistic, as opposed to realistic novels that focused on the external world.

Impressionistic literature can be defined as a work created by an author that centers on the thinking and feelings of the characters and allows the reader to draw his or her own interpretations and conclusions about their meaning.

Absolutely, Heart of Darkness is often cited as one of the preeminent examples of Impressionistic literature. Throughout the novella, we witness the inner workings of Marlow's thoughts and emotions as he journeys up the Congo River toward the Inner Station and his encounter with Kurtz. In fact, it is Kurtz who presents the major focus on Marlow's attention. However, Marlow never comes out and explicitly tells us what he thinks and believes about Kurtz; instead, Conrad leaves us to draw our own conclusions.

Kurtz, too, is much the same. He also shares his thoughts and feelings about his job and his role in the African jungle and the reader is left to "read between the lines" to interpret his exact meaning. No example could better exemplify the element of Impressionism with the novella than Kurtz's final words: "the horror, the horror." A readers we do not know exactly what "the horror" is and instead are left to devise our own interpretation.