Douglas Harper's Etymology Dictionary
"three successive lines rhyming together," 1590s, from Italian terzetto, diminutive of terzo "third," from Latin tertius (see third). Spelling influenced by French tercet, from the Italian.
The Collaborative International Dictionary
Tercet \Ter"cet\, n. [F., fr. It. terzetto, dim. of terzo, third, L. tertius. See Tierce, and cf. Terzetto.]
(Mus.) A triplet.
(Poetry) A triplet; a group of three lines.
n. a three-line stanza in a poem
Usage examples of "tercet".
It has fourteen lines that divide into an octave of a rhyme scheme ABBA ABBA and a sestet CDC DCD, really two tercets.
A vague idea had stirred while I was reading Louis MacNeice's long poem ``Autumn Journal'' written in tercets, but it was while reading Part II of T.
Eliot's ``Little Gidding'' that it struck me that, though unrhymed, the poem was also written in tercets, the form favoured by Dante.