The meaning of Martyr in the Bible
(From International Standard Bible Encyclopedia)

mar'-ter (martus, Aeolic martur): One who gives heed, and so, a "witness," so translated in numerous passages, both as of one bearing testimony, and also as of one who is a spectator of anything (see WITNESS). In the King James Version rendered "martyr" in Acts 22:20, "thy martyr Stephen"; and Revelation 2:13, "Antipas my faithful martyr"; also Revelation 17:6, "the blood of the martyrs of Jesus," where alone the American Standard Revised Version retains "martyrs." These 3 passages are the beginning of the use of the word "martyr" for such witnesses as were faithful even unto death, its uniform modern use.

See more on the meaning of Martyr in the Bible:
Martyr {Easton's Bible Dictionary}

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