A tercet is three lines of poetry forming a stanza or complete poem. Haiku is an example of an unrhymed tercet poem.
Other types of tercet include enclosed tercet where the lines rhyme in an a b a pattern and terza rima where the a b a pattern of a verse is continued in the next verse by rhyming the unrhymed line. The pattern for this is: aba bcb cdc ded...