Tercet
A three-line stanza — the sonnet's closing unit and, chained as terza rima, Dante's vehicle.
The tercet is a three-line stanza. Its odd number gives it a particular dynamic: where the quatrain balances, the tercet leans — calling for a continuation or precipitating an end. That's why it occupies the sonnet's second half in the French tradition: after the setting quatrains, the two tercets pivot and conclude.
Chained in braided rhymes (ABA BCB CDC…), it becomes terza rima, Dante's invention for the Divine Comedy: each tercet contains the seed of the next, and the poem advances like a chain with no possible end — a single line is needed to stop it. As an autonomous brief form, the tercet is also the haiku's Western costume.
Example
Dante's terza rima: ABA BCB CDC… — the middle rhyme begets the next stanza.