Terzain is an old term for a three-line stanza, the same form more commonly called a tercet or triplet. The three lines may share a rhyme scheme or may be left unrhymed, depending on the poem and tradition.
The word is uncommon in modern poetry study, so readers will usually encounter tercet instead.
Example
Soft rain on the window
A kettle starts to murmur
Night settles in the room