Monks were like squirrels... they'd find any piece of music and scribble it down for later consumption, stealing them and filing the serial numbers off. Many an old hymn tune started out as a raunchy folk song.

'Sacred' music = Any music that is part of the Holy Sacrament (Eucharist, Communion, Mass) or refers to a member of the Christian Trinity, or is taken from Holy Scripture.

'Religious' = Any music that praises or narrates all the great things the Trinity has done.

'Secular' = Any music that does not immediately pertain to the Christian religion, usually about pretty fair maids walking in May, what ploughboys get up to and how badly the new landlord is treating you.

LTS - gleaned from ancient notes taken in an RSCM workshop!