introit
noun
in·troit ˈin-ˌtrȯit 
-ˌtrō-ət,
in-ˈtrȯit,
-ˈtrō-ət
1
often capitalized : the first part of the traditional proper of the Mass consisting of an antiphon, verse from a psalm, and the Gloria Patri
2
: a piece of music sung or played at the beginning of a worship service
Word History
Etymology
Middle English, from Medieval Latin introitus, from Latin, entrance, from introire to go in, from intro- + ire to go — more at issue entry 1
First Known Use
15th century, in the meaning defined at sense 1