The Collaborative International Dictionary of English v.0.48:

Mass \Mass\ (m[.a]s), n. [OE. masse, messe, AS. maesse. LL.
   missa, from L. mittere, missum, to send, dismiss: cf. F.
   messe. In the ancient churches, the public services at which
   the catechumens were permitted to be present were called
   missa catechumenorum, ending with the reading of the Gospel.
   Then they were dismissed with these words : "Ite, missa est"
   [sc. ecclesia], the congregation is dismissed. After that the
   sacrifice proper began. At its close the same words were said
   to those who remained. So the word gave the name of Mass to
   the sacrifice in the Catholic Church. See Missile, and cf.
   Christmas, Lammas, Mess a dish, Missal.]
   [1913 Webster]
   1. (R. C. Ch.) The sacrifice in the sacrament of the
      Eucharist, or the consecration and oblation of the host.
      [1913 Webster]

   2. (Mus.) The portions of the Mass usually set to music,
      considered as a musical composition; -- namely, the Kyrie,
      the Gloria, the Credo, the Sanctus, and the Agnus Dei,
      besides sometimes an Offertory and the Benedictus.
      [1913 Webster]

   Canon of the Mass. See Canon.

   High Mass, Mass with incense, music, the assistance of a
      deacon, subdeacon, etc.

   Low Mass, Mass which is said by the priest throughout,
      without music.

   Mass bell, the sanctus bell. See Sanctus.

   Mass book, the missal or Roman Catholic service book.
      [1913 Webster]

WordNet (r) 3.0 (2006):

Low Mass
    n 1: a Mass recited without music