Tercet: Broadly speaking, a group of three lines of verse. So used, tercet may apply to a poem, a stanza, or a part of a stanza, and it may also apply regardless of rhyme scheme. Thus, the term may encompass a wide variety of forms, including the haiku, an unrhymed, three-line poem; the closing, usually unrhymed three-line stanza of the sestina; the aba stanzas of the villanelle; the interlocking stanzas of terza rima, which rhyme aba bcb etc.; and the two three-line components of the sestet, which often rhyme cdecde, in the Italian sonnet. A tercet in which all three lines rhyme (aaa) is often called a triplet.
EXAMPLE: The following stanza, one of six tercets in Trumbull Stickney’s “Mnemosyne” (1902):
I had a sister lovely in my sight:
Her hair was dark, her eyes were very sombre;
We sang together in the woods at night.
Adrienne Rich’s “Terza Rima” (2000) contains thirteen numbered sections written almost entirely in tercets. Susan Mitchell’s “Dragonfly” (2007) consists of sixteen unrhymed tercets.