Prose
The word "prose" first appears in English in the 14th century. It is derived from the Old French "prose", which in turn originates in the Latin expression "prosa oratio" (literally, straightforward or direct speech).
Eureka: A Prose Poem
Poe's decision to refer to the piece as a "prose poem" goes against some of his own "rules" of poetry which he laid out in "The Philosophy of Composition" and "The Poetic Principle". In particular, Poe called the ideal poem short, at most 100 lines, and utilizing the "most poetical topic in the world": the death of a beautiful woman. Poe himself suggested that the work be judged only as a work of art, not of science, possibly dismissing the seriousness of "Eureka". Though he is using mathematical and scientific terms, he may really be talking about aesthetics and suggesting there is a close connection between science and art. This is an ironic sentiment when compared to his message in the poem "To Science" where he shows a distaste for modern science encroaching on spirituality and the artist's imagination. Poe also discusses several astronomy-related topics in "Eureka", including the speed of the stars, the diameters of planets and distance between them, the weight of Earth, and the orbit of the newly discovered "Leverrier's planet" (later named Neptune).