Definition and background:
Chants and prayers from the Mass and other services that remain the same from day to day throughout the Church year. The ordinary is opposed to the proper, which consists of those chants and prayers that vary from day to day according to the particular liturgical occasion. Typical Ordinary Mass Typical Proper Mass Introit Kyrie eleison Kyrie eleison Gloria in excelsis Deo Gloria in excelsis Deo Gradual Alleluia Sequence Credo Credo Offertory Sanctus Sanctus Benedictus Benedictus Agnus Dei Agnus Dei Communion Ite missa est Ite missa est a liturgical genre is ordinary if its text is repeated from day to day. In the mass, the musical items of the ordinary are the Kyrie, Gloria, Credo, Sanctus, and Agnus dei; the Ite missa est and the Benedicamus Domino may also be considered ordinary. A number of different melodies are associated with each ordinary text, so while the text repeats, the music may change from one day to the next. A table of items from the mass is available. in the mass, the parts that are used every day, as distinct from the proper. the ordinary consists of the kyrie, gloria, credo, sanctus, and agnus dei.
For more information on the music term "Ordinary" check out these other resources:
Wikipedia - Glossary of Musical Terminology
Virginia Tech Multimedia Music Dictionary
ORB -- Medieval Music Glossary