How do we define the term Gothic, and what Gothic aspects can be found in the fiction of Edgar Allan Poe?
One of the most dominant motifs in Gothic literature is decay. Gothic literature portrays the world in a fallen state, and it is through the sense of decay that this fallen state is illustrated. The fallen, decaying world can be illustrated through scenery, plot, characterization, and even theme. The protagonist will also represent a fallen state by being "isolated either voluntarily or involuntarily" (UC Davis, "The Gothic Novel"). The works of Edgar Allan Poe are full of decay through references to death and even the supernatural. Even illusions to misdeeds of the protagonists will characterize the protagonists in their isolated, fallen state.
Some examples can easily be seen in his short story "The Tell-Tale Heart." Here, the protagonist is characterized from the start as a madman: "True!--nervous--very, very dreadfully nervous I had been and am; but why will you say that I am mad?" Being mad is certainly not the ideal state, so we can easily see how this depicts a character in a fallen state; plus, madmen are isolated from society, so he is also being characterized as isolated. Moreover, his fallen state is characterized by his description of his senseless murder of the old man who was his housemate. Even the very reference to murder, which is death, is clearly a reference to the world in a state of decay.
In Gothic art and literature, as distinct from Gothic architecture, an emphasis is placed on the intensity of emotional responses to landscapes and specific places and, more specifically, on the impact of these place on an individual's psychological health. In the most extreme cases (and the most powerful Gothic representations), the impact is profound, with individuals declining into madness.
This Gothic decline is very much evident throughout Poe's work. In his short fiction, for instance, Roderick is defined by the acuity of his senses and his sensitivity to both light and noises within the decaying family home in which they live (and are trapped). The physical sensitivity is aligned clearly with his obsession with his dying sister, which erupts into full madness when he kills her at the story's denouement (ending).