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