"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.
a unit or group of three lines of verse, usually containing rhyme, as in William Shakespeare's "The Phoenix and the Turtle": Death is now the phoenix' nest; And the turtle's loyal breastTo eternity doth rest,