Missa brevis usually refers to a mass composition that is short because part of the text of the Mass ordinary that is usually set to music in a full mass is left out, or because its execution time is relatively short.
Start of Gaspar van Weerbeke's Missa brevis in Choirbook, D-Ju MS 21
Start of the Kyrie of Johann Sebastian Bach's Mass in B minor, originally composed as the start of a Kyrie–Gloria Mass in B minor dedicated to Frederick Augustus II, Elector of Saxony, when he came to power in 1733. The original Kyrie–Gloria mass was composed in 12 movements for SSATB soloists and choir, and an extensive baroque orchestra. It was probably because of its long duration that the score was archived in the Royal Library upon arrival in Dresden, instead of being added to the repertoire of the Catholic Hofkirche.
The Mass is a form of sacred musical composition that sets the invariable portions of the Christian Eucharistic liturgy, known as the Mass.
Missa Virgo parens Christi by Jacobus Barbireau