From 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]

From 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]

From WordNet (r) 2.0:

martyr
     n 1: one who suffers for the sake of principle [syn: 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: martyrize, martyrise]

From Moby Thesaurus II by Grady Ward, 1.0:

118 Moby Thesaurus words for "martyr":
   afflict, agonize, ail, angel, angel of light, angel of love,
   annihilate, archangel, beatified soul, bereave of life, bite,
   bloody, burn, canonized mortal, carry away, carry off, celestial,
   chafe, cherub, cherubim, chloroform, claw, convulse, crucify, cut,
   cut down, cut off, deprive of life, destroy, dispatch, dispose of,
   distress, do away with, do for, do to death, end, excruciate,
   execute, exterminate, fester, finish, finish off, fret, gall,
   give pain, gnaw, grate, grind, gripe, harrow, heavenly being, hurt,
   immolate, impale, inflame, inflict pain, irritate, kill,
   kill by inches, lacerate, lancinate, launch into eternity,
   liquidate, lynch, macerate, make away with, martyrize,
   messenger of God, nip, object of compassion, pain, patron saint,
   pierce, pinch, poison, poor devil, prey, prick, principality,
   prolong the agony, punish, purge, put away, put down, put to death,
   put to sleep, put to torture, rack, rankle, rasp, recording angel,
   remove from life, rip, rub, sacrifice, saint, savage, saved soul,
   scarify, seraph, seraphim, slay, soul in glory, stab, starve,
   sting, sufferer, take life, take off, torment, torture, try, tweak,
   twist, victim, wound, wretch, wring

From Easton's 1897 Bible Dictionary:

Martyr
   one who bears witness of the truth, and suffers death in the
   cause of Christ (Acts 22:20; Rev. 2:13; 17:6). In this sense
   Stephen was the first martyr. The Greek word so rendered in all
   other cases is translated "witness." (1.) In a court of justice
   (Matt. 18:16; 26:65; Acts 6:13; 7:58; Heb. 10:28; 1 Tim. 5:19).
   (2.) As of one bearing testimony to the truth of what he has
   seen or known (Luke 24:48; Acts 1:8, 22; Rom. 1:9; 1 Thess. 2:5,
   10; 1 John 1:2).
   

From THE DEVIL'S DICTIONARY ((C)1911 Released April 15 1993):

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