The cywydd (in Welsh pronounced as /ˈkəwɨ̞ð/; plural Welsh: cywyddau) is one of the most important metrical forms in traditional Welsh poetry (cerdd dafod).
There are a variety of forms of the cywydd, but the word on its own is generally used to refer to the Welsh: cywydd deuair hirion ("long-lined couplet") as it is by far the most common type.
The first recorded examples of the cywydd date from the early 14th century, when it is believed to have been developed. This was the favourite metre of the Poets of the Nobility from the 14th to 17th centuries and is still used in the 21st.
The cywydd consists of a series of seven-syllable lines in rhyming couplets, with all lines written in cynghanedd. One of the lines must finish with a stressed syllable, while the other must finish with an unstressed syllable. The rhyme may vary from couplet to couplet, or may remain the same. There is no rule about how many couplets there must be in a cywydd.
The Welsh: cywydd deuair hirion and the related Welsh: cywydd deuair fyrion, Welsh: cywydd llosgyrnog and the Welsh: awdl-gywydd all occur in the list of the twenty four traditional Welsh poetic meters adopted in the later Middle Ages.