POETRY
I. Means and Ends. A poem is an instance of verbal art, a text set in verse, bound speech… Traditionally these have been taken as the ones offered by pros., i.e. verse form: lineation, meter, sound-pattering, syntactic deployment, and stanza forms…
p. has traditionally been distinguished from prose by virtue of being set in verse. What most readers understand as p. was, up until 1850, set in lines which were metrical, and even the several forms of vers libre and free verse produced since 1850 have been built largely on one or another concept of the line.
Lineation is therefore central to the traditional conception of p. Prose is cast in sentences; p. is cast in sentences cast into lines. Prose syntax has the shape of meaning, but poetic syntax is stretched across the frame of meter or the poem’s visual space, so that it has this shape as well as meaning. Whether the pros. of the poem is primarily aural, visual, or mixed it creates design. (p. 938-939)
I could also cite Furniss/Bath’s textbook, and many others that mention “line” as the unifying element of all objects we call poetry.