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]

The Collaborative International Dictionary of English v.0.48:

mastered \mastered\ adj.
   Learned thoroughly.

   Syn: down, down pat(predicate).
        [WordNet 1.5]

WordNet (r) 3.0 (2006):

mastered
    adj 1: understood perfectly; "had his algebra problems down"
           [syn: down, down pat(p), mastered]