"Gloria in excelsis Deo" (Latin for "Glory to God in the highest") is a hymn known also as the Greater Doxology (as distinguished from the "Minor Doxology" or Gloria Patri) and the Angelic Hymn. The name is often abbreviated to Gloria in Excelsis or simply Gloria.
It is an example of the psalmi idiotici ("private psalms", i.e. compositions by individuals in imitation of the biblical Psalter) that were popular in the 2nd and 3rd centuries. Other surviving examples of this lyric poetry are the Te Deum and the Phos Hilaron.
Read more about Gloria In Excelsis Deo: History, Present-day Greek Text, Present-day Latin Text, Musical Settings, Some English Translations