What is a quatrain? an unstressed syllable followed by a stressed syllable two lines of verse that restate the theme a four-line stanza?

A quatrain is a verse with four lines, or even a full poem containing four lines, having an independent and separate theme. Often one line consists of alternating rhyme, existing in a variety of forms. We can trace back quatrains in poetic traditions of various ancient civilizations, such as China, Ancient Rome, and Ancient Greece; and they continue to appear in the twenty-first century.