|There is more information available on this subject at Gregorian Chant on the English Wikipedia.|
Gregorian Chant is the central tradition of Western plainchant, a form of monophonic liturgical chant in Western Christianity that accompanied the celebration of Mass and other ritual services.
This vast repertory of chants is the oldest music known as it is the first repertory to have been adequately notated in the 10th century. In general, the chants were learned by the viva voce method, that is by following the given example orally, which took many years of experience in the Schola Cantorum. Gregorian chant originated in Monastic life, in which singing the 'Divine Service' nine times a day at the proper hours was upheld according to the Rule of St. Benedict. Singing psalms made up a large part of the life in a monastic community, while a smaller group and soloists sang the chants. In its long history Gregorian Chant has been subjected to many gradual changes and some reforms.
A Gregorian chant is often used as the theme music for the Halo Installations in the Halo series, likely alluding to the strong religious connotations they possess for the Covenant, who regard them as relics left behind by their gods, the species that built them.