What term is used for a six-line stanza, commonly associated with the second division of an Italian sonnet?

The term for a six-line stanza, particularly in the context of an Italian sonnet, is indeed "sestet." In the structure of an Italian sonnet, which is typically composed of an octave (the first eight lines) followed by a sestet, the sestet serves to resolve or comment on the problem or theme presented in the octave. It often explores the implications, offers a turn in thought, or provides a resolution to the tension built up in the first part of the sonnet.

The other terms refer to different stanza forms: a quatrain consists of four lines and is commonly found in various types of poetry, an octave consists of eight lines (the first part of the Italian sonnet), and a couplet is a pair of lines, usually rhymed, which can stand alone or be part of a longer poem. Thus, the correct understanding of the sestet is essential for appreciating the structure and function of the Italian sonnet in poetic literature.