The Collaborative International Dictionary of English v.0.48:

Martyr \Mar"tyr\, n. [AS., from L. martyr, Gr. ma`rtyr, ma`rtys,
   prop., a witness; cf. Skr. sm[.r] to remember, E. memory.]
   [1913 Webster]
   1. One who, by his death, bears witness to the truth of the
      gospel; one who is put to death for his religion; as,
      Stephen was the first Christian martyr. --Chaucer.
      [1913 Webster]

            To be a martyr, signifies only to witness the truth
            of Christ; but the witnessing of the truth was then
            so generally attended with persecution, that
            martyrdom now signifies not only to witness, but to
            witness by death.                     --South.
      [1913 Webster]

   2. Hence, one who sacrifices his life, his station, or what
      is of great value to him, for the sake of principle, or to
      sustain a cause.
      [1913 Webster]

            Then if thou fall'st, O Cromwell,
            Thou fall'st a blessed martyr !       --Shak.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Martyr \Mar"tyr\, v. t. [imp. & p. p. Martyred; p. pr. & vb.
   n. Martyring.]
   1. To put to death for adhering to some belief, esp.
      Christianity; to sacrifice on account of faith or
      profession. --Bp. Pearson.
      [1913 Webster]

   2. To persecute; to torment; to torture. --Chaucer.
      [1913 Webster]

            The lovely Amoret, whose gentle heart
            Thou martyrest with sorrow and with smart.
                                                  --Spenser.
      [1913 Webster]

            Racked with sciatics, martyred with the stone.
                                                  --Pope.
      [1913 Webster]

WordNet (r) 3.0 (2006):

martyr
    n 1: one who suffers for the sake of principle [syn: martyr,
         sufferer]
    2: one who voluntarily suffers death as the penalty for refusing
       to renounce their religion
    v 1: kill as a martyr; "Saint Sebastian was martyred"
    2: torture and torment like a martyr [syn: martyr,
       martyrize, martyrise]

The Devil's Dictionary (1881-1906):

MARTYR, n.  One who moves along the line of least reluctance to a
desired death.