What is the meaning of 'excelsis'?

"In Excelsis" is Latin for 'in the highest'. The phrase is found in many Christian expressions in the New Testament. It is also a part of the ordinary mass, and is the title of many major classical productions. Vivaldi, Bach,, Mozart and others have musical masses in which a significant section is entitled the Excelsis, or Gloria in Excelsis deo. YouTube has many performances of the Gloris, as part of major musical productions through the last few hundred years.