In its original meaning, the word martyr, meaning witness, was used in the secular sphere as well as in the New Testament of the Bible. The process of bearing witness was not intended to lead to the death of the witness, although it is known from ancient writers (e.g. Josephus) and from the New Testament that witnesses often died for their testimonies.
During the early Christian centuries, the term acquired the extended meaning of a believer who is called to witness for their religious belief, and on account of this witness, endures suffering and/or death. The term, in this later sense, entered the English language as a loanword. The death of a martyr or the value attributed to it is called martyrdom. 
- 1: a person who voluntarily suffers death as the penalty of witnessing to and refusing to renounce a religion
- 2: a person who sacrifices something of great value and especially life itself for the sake of principle
- 3: victim; especially : a great or constant sufferer <a martyr to asthma all his life — A. J. Cronin>