Martyr is from the Greek μαρτυρ (sometimes rendered in English letters as "martureo"). This is the Greek word for "witness". Sound familiar?
But you will receive power when the Holy Spirit comes on you; and you will be my witnesses in Jerusalem, and in all Judea and Samaria, and to the ends of the earth.The focus here in naming Stephen a martyr is not on his death alone, but that he could not be disuaded from his witness about Christ by anything - not persecution, not suffering, not even death itself. He was truly faithful to his Lord, and this is why he is to be remembered as an example to all believers.
Acts 1:8