prose is written or spoken language in its ordinary form, following the natural rhythms of speech rather than the metrical structure of verse.
The useful contrast is with verse, not with poetry. Verse is language organized into measured lines; poetry is an art that usually but not necessarily uses verse to achieve its effects. The prose poem exists precisely in that gap, and a metrical 📝epic and a novel differ in form long before they differ in ambition. A second and older sense of prose is evaluative rather than formal: from the 1680s the word carried the derogatory charge of dull or commonplace expression, a sense that survives in prosaic and in the now-rare verb to prose, meaning to write or speak tediously.
The etymology encodes the contrast the word was coined to make. Prose entered English around 1300 from Old French prose, from Latin prosa, itself shortened from prosa oratio, "straightforward speech," on the adjective prosus — earlier prorsus — meaning "straight ahead," from pro-, "forward," plus vorsus, "turned," from vertere, "to turn." Verse comes from the same verb by the opposite image: Latin versus named the turn a plow makes at the end of a furrow, and then the line of writing that turns back at its end. Prose runs forward without turning; verse turns. In its earliest English use the word simply meant story or narration, taking on the modern sense of non-poetic writing by the mid-fourteenth century.