Intonation (music)
In plainchant, intonation is when a solo singer - usually the priest - sings the first phrase of a piece of music before the choir start to sing.  The intonation is the tune on which the music is based.  For example, when the choir sings a piece called Gloria the priest first sings "Gloria in excelsis Deo" ("Glory be to God") by himself, unaccompanied.  Then the choir continue singing with the words "et in terra pax....." ("and peace on earth....).