There are 4 particular definitions of mass that I believe are especially important to note. (Sorry this is coming in a little later.)

1. Mass has a primary usage regarding the celebration of the Eucharist during a Roman Catholic service.
   1.  The Eucharist, esp. (in post-Reformation use) as understood and celebrated in the Roman Catholic Church.In the 16th cent. many Protestants objected to the term as unscriptural, and as being associated with the ‘popish’ view of the nature of the sacrament. The German liturgy formulated by Martin Luther in 1526 was, however, called the Deutsche Messe, and Swedish mässa and Danish messe are applied to the Lutheran rite. In the first Book of Common Prayer of Edward VI (1548–9) the heading of the service reads ‘The Holy Communion, commonly called the Masse’, but in the subsequent Prayer Books the word was not used.
2. Mass also relates to the bodily form or just objects with density in general.
   1. A body of matter, and related senses.
      1. A dense aggregation of objects having the appearance of a single, continuous body.
   2. A coherent body of matter of unspecified or indeterminate shape, and usually of relatively large bulk; a solid and distinct object occupying space.
3. Mass has also historically been utilized as a verb denoting the actual celebration.
   1. To celebrate mass; to say or sing mass.
      1. To hear or attend mass.
4. Akin to the object definition, mass can be used as a verb denoting collection.
   1. To form or gather into a mass; to collect, arrange, or bring together in masses; to amass.

      All definitions were gathered from the OED.