NOUN (2)

1. becoming proficient in the use of something; having mastery of;
- Example: "his mastering the art of cooking took a long time"

2. the act of making a master recording from which copies can be made;
- Example: "he received a bill for mastering the concert and making 100 copies"

The Collaborative International Dictionary of English v.0.48:

Master \Mas"ter\, v. t. [imp. & p. p. Mastered; p. pr. vb. n.
   Mastering.]
   1. To become the master of; to subject to one's will,
      control, or authority; to conquer; to overpower; to
      subdue.
      [1913 Webster]

            Obstinacy and willful neglects must be mastered,
            even though it cost blows.            --Locke.
      [1913 Webster]

   2. To gain the command of, so as to understand or apply; to
      become an adept in; as, to master a science.
      [1913 Webster]

   3. To own; to posses. [Obs.]
      [1913 Webster]

            The wealth
            That the world masters.               --Shak.
      [1913 Webster]

WordNet (r) 3.0 (2006):

mastering
    n 1: becoming proficient in the use of something; having mastery
         of; "his mastering the art of cooking took a long time"
    2: the act of making a master recording from which copies can be
       made; "he received a bill for mastering the concert and
       making 100 copies"