In the Western Church, the Mass (or Eucharist, or Lord’s Supper) is the service of thanksgiving in which the Church remembers and gives thanks for Christ’s sacrificial act of Love in his death on the Cross. The service re-enacts the Last Supper that Jesus shared with his disciples, and goes right back to the earliest days of the Church’s existence.

In the Catholic tradition, it is the regular act of the Church’s worship, being celebrated on a daily basis as well as being the focus of Sunday worship.

In the context of the title of a musical work, it refers to the musical setting of five key texts (the ‘Ordinarium’) within the service that don’t change with the seasons in the Church’s year, and that have traditionally been set to music and sung. They are known as follows, after the initial words of the Latin or Greek text:

• Kyrie eleison (‘Lord, have mercy…’)
• Gloria in excelsis Deo (‘Glory to God on high…’)
• Credo in unum Deum (‘I believe in one God…’)
• Sanctus and Benedictus (‘Holy, Holy, Holy…’ )
• Agnus Dei (‘Lamb of God…’)

The term Missa (which means Mass) sometimes refers to just the Kyrie and Gloria, as in some contexts such as the Lutheran Church or the Court at Dresden in the 1730s these were the only sections used.

Full Latin text and English translation

Further reading:

Mass in the Catholic Church (Wikipedia)

The Eucharist (Britannica)