Stanza
A group of lines forming a unit, separated from others by white space — the poem's paragraph.
The stanza groups several lines into a visual and sonic unit, isolated by typographic white space. Regular stanzas have names by size: couplet (2 lines), tercet (3), quatrain (4), quintet (5), sestet (6)… In fixed forms their arrangement is codified — octave plus sestet make the Petrarchan sonnet.
The stanza is to the poem what the paragraph is to prose, with one extra power: the white space after it is real time, a measurable silence the poet composes as much as the lines. In free verse, the stanza becomes the major breathing unit — regular, it installs a ritual; irregular, every format break becomes an event.
Example
Three regular quatrains, then a single isolated line: the final solitude makes the poem.