What is "twin quasar"

n. (context galaxy English) a pair of quasar images formed by gravitational lensing of a single physical quasar

The Twin Quasar (Twin QSO or Double Quasar or Old Faithful, also known as SBS 0957+561, TXS 0957+561, Q0957+561 or QSO 0957+561 A/B), was discovered in 1979 and was the first identified gravitationally lensed object. It is a quasar that appears as two images, a result from gravitational lensing caused by the galaxy YGKOW G1 that is located in the line of sight between Earth and the quasar.