Radif (Arabic: رديف) is a rule in Persian, Turkic, and Urdu poetry which states that, in the form of poetry known as a Ghazal, the second line of all the couplets (bayts or Shers) must end with the same word/s. This repeating of common words is the "Radif" of the Ghazal. It is proceeded by a Qaafiyaa, which is a repeating pattern of words.
The following is an example of a Ghazal by Daag Dehelvi. In this example the Radif is mein. The Qaafiyaa is the following pattern of words: nigaah (in the first hemistich), jalwa-gaah, nigaah (in the fourth hemistich), raah, haale-tabaah and aah.
aafat kii shoKhiyaa.N hai.n tumhaarii nigaah mein
mehashar ke fitane khelate hai.n jalwa-gaah mein
wo dushmanii se dekhate hai.n dekhate to hain
mai.n shaad huu.N ke huu.N to kisii kii nigaah mein
aatii baat baat mujhe yaad baar baar
kahataa huu.N dau.D dau.D ke qaasid se raah mein
is taubaah par hai naaz mujhe zaahid is qadar
jo TuuT kar shariik huu.N haal-e-tabaah mein
mushtaaq is adaa ke bahot dard-ma.nd the
ae DaaG tum to baiTh gaye ek aah mein