In Christianity, a martyr is a person who was killed for their testimony for Jesus or faith in Jesus. In the years of the early church, stories depict this often occurring through death by sawing, stoning, crucifixion, burning at the stake, or other forms of torture and capital punishment. The word martyr comes from the Koine word μάρτυς, mártys, which means "witness" or "testimony".

At first, the term applied to the Apostles. Once Christians started to undergo persecution, the term came to be applied to those who suffered hardships for their faith. Finally, it was restricted to those who had been killed for their faith. The early Christian period before Constantine I was the "Age of Martyrs". "Early Christians venerated martyrs as powerful intercessors, and their utterances were treasured as inspired by the Holy Spirit."

In western Christian art, martyrs are often shown holding a palm frond as an attribute, representing the victory of spirit over flesh, and it was widely believed that a picture of a palm on a tomb meant that a martyr was buried there.

The use of the word μάρτυς (mártys) in non-biblical Greek was primarily in a legal context. It was used for a person who speaks from personal observation. The martyr, when used in a non-legal context, may also signify a proclamation that the speaker believes to be truthful. The term was used by Aristotle for observations, but also for ethical judgments and expressions of moral conviction that can not be empirically observed. There are several examples where Plato uses the term to signify "witness to truth", including in Laws.

The Greek word martyr signifies a "witness" who testifies to a fact he has knowledge about from personal observation. It is in this sense that the term first appears in the Book of Acts, in reference to the Apostles as "witnesses" of all that they had observed in the public life of Christ. In Acts 1:22, Peter, in his address to the Apostles and disciples regarding the election of a successor to Judas, employs the term with this meaning: "Wherefore, of these men who have accompanied with us all the time that the Lord Jesus came in and went out among us, beginning from the baptism of John until the day he was taken up from us, one of these must be made a witness with us of his resurrection".

The Apostles, according to tradition, faced grave dangers until eventually almost all suffered death for their convictions. The Bible reports the martyrdom of two of the apostles. Thus, within the lifetime of the Apostles, the term martyrs came to be used in the sense of a witness who at any time might be called upon to deny what he testified to, under penalty of death. From this stage the transition was easy to the ordinary meaning of the term, as used ever since in Christian literature: a martyr, or witness of Christ, is a person who suffers death rather than deny his faith. Saint John, at the end of the first century, employs the word with this meaning. A distinction between martyrs and confessors is traceable to the latter part of the second century: those only were martyrs who had suffered the extreme penalty, whereas the title of confessor was given to Christians who had shown their willingness to die for their belief, by bravely enduring imprisonment or torture, but were not put to death. Yet the term martyr was still sometimes applied during the third century to persons still living, as, for instance, by Cyprian who gave the title of martyrs to a number of bishops, priests, and laymen condemned to penal servitude in the mines.

Religious martyrdom is considered one of the more significant contributions of Second Temple Judaism to western civilization. It is believed that the concept of voluntary death for God developed out of the conflict between King Antiochus Epiphanes IV and the Jewish people. 1 Maccabees and 2 Maccabees recount numerous martyrdoms suffered by Jews resisting the Hellenizing of their Seleucid overlords, being executed for such crimes as observing the Sabbath, circumcising their children, or refusing to eat pork or meat sacrificed to foreign gods. With few exceptions, this assumption has lasted from the early Christian period to this day, accepted both by Jews and Christians.

According to Daniel Boyarin, there are "two major theses with regard to the origins of Christian martyrology, which [can be referred to] as the Frend thesis and the Bowersock thesis". Boyarin characterizes W. H. C. Frend's view of martyrdom as having originated in Judaism and Christian martyrdom as a continuation of that practice. Frend argues that the Christian concept of martyrdom can only be understood as springing from Jewish roots. Frend characterizes Judaism as "a religion of martyrdom" and that it was this "Jewish psychology of martyrdom" that inspired Christian martyrdom. Frend writes, "In the first two centuries AD. there was a living pagan tradition of self-sacrifice for a cause, a preparedness if necessary to defy an unjust ruler, that existed alongside the developing Christian concept of martyrdom inherited from Judaism."