The Latin origin of the word prose (family of "prodeo" and "prorsus") defines its meaning: "prosa oratio" is the lyrical expression of the thing, whether written or oral, which is moving forward as opposed to verse that overturns in a choice of rhythmic note layout. "Anything that is not prose is verse, and everything that is not verse is prose" an extract from Molière, Le Bourgeois Gentilhomme, Act II, Scene IV.
Applicable to the common expression, the word prose can have a pejorative meaning (for example: "Administrative prose"). It also occurs in a metaphorical use to describe something mundane and flat, and this expresses the word "prosaic". The prose used in all literary genres, of course can seem like a real stylistic work up to the poetic prose as in Rousseau, Chateaubriand and Giono.
Poetic prose
The poetic prose however remained, an additional means for the novelist to create a trademark of his style, while not being a true form of poem. Prose literature has been popular for centuries, the most known being French authors. Around 1800, while the romance is formed, the aspirations of writers tend to lean more toward the absolute. Poetry arouses new interest (as opposed to the Enlightenment when it was considered an ornament) and versification was relaxed, especially by Victor Hugo.
However this is not enough for some temperaments, who undergo more difficultly the tyranny of rhyme and meter. Prose translations of poems from across the handle assigned to the mythical Ossian will be followed by several short texts written by poetic authors less known today (Xavier Forneret Jules Lefevre-Deumier). Around the same time Maurice de Guerin wrote Le Centaure, lyrical prose text along a dozen of pages.
It was in this climate that in 1842 Aloysius Bertrand published Gaspard de la Nuit, considered the seminal book of its kind in France. He consciously used the shape of the medieval ballad in proses to evoke the dreamlike or fantastic scenes, rather than printing stories. We consider the author as the real creator of the poem in prose.