A tercet is a poem or a stanza with three lines. English-language haiku is one example of a simple, non-rhyming tercet.
A haiku is a stand-alone tercet, a complete poem in three lines (and, often, 17 syllables). Other poems are broken into stanzas, each of which is a tercet. Tercets frequently rhyme in an ABA pattern, like the stanzas in Shelley's Ode to the West Wind: lines of the first tercet end with "being," "dead," and "fleeing," and the next with "red," "thou," and "bed." The Latin root of tercet means "third."