From Greek 'martys' (witness) — transformed by early Christians into one who witnesses faith through suffering and death.
A person who suffers death or great suffering for a cause, belief, or principle; originally, one who dies rather than renounce their religious faith.
From Old English 'martyr,' borrowed from Ecclesiastical Latin 'martyr,' from Greek 'martys' (μάρτυς, genitive 'martyros'), meaning a witness, one who testifies to what they have seen or known. In classical Greek, 'martys' had no connection to death or suffering — it was a legal and rhetorical term for an eyewitness, equivalent to Latin 'testis.' The word's deeper etymology is disputed: some scholars connect it to PIE *mer- (to remember, to be mindful of), which produced Sanskrit 'smarati' (he remembers
Greek 'martys' (witness) is likely related to Proto-Indo-European *smer- (to remember), the same root that produced Sanskrit 'smarati' (he remembers) and English 'memory.' A witness, at the deepest etymological level, is one who remembers — making a martyr someone whose remembrance costs them everything.