Agnus Dei is the fifth part in the Latin Mass Ordinary. It is taken from the text of John i, 29 (Behold the Lamb of God) and, like much of the rite, was first set to music as plainchant in the seventh century before following the rest of musical liturgy through the trends and fashions of musical history.
It is sung during the breaking of bread.
Unlike certain other parts of the Mass it has remained within its context, rarely straying outside its liturgical boundaries and taking on a life of its own.. However, one of the arrangements that Samuel Barber made of that movement of his String Quartet, best known as “Adagio for Strings”, is an unaccompanied choral version of “Agnus Dei”. https://www.youtube.com/watch?v=mWgjwTc3PUw