A pause that occurs in the middle of lines, either grammatical or rhetorical. It's used to vary rhythm in poetry.

From Walt Whitman's A Noiseless Patient Spider:

    It launched forth filament,|| filament,|| filament,|| out of itself, (Grammatical)
From William Carlos Williams' The Widow's Lament in Springtime:
    flames || as it has flamed
    often before || but not