Martus (μάρτυς) originally meant a witness — one who gives testimony based on firsthand knowledge or experience. In the NT it describes the apostolic eyewitnesses of the resurrection, and then, as witnessing increasingly led to death, the word evolved to mean 'martyr' — one who bears witness unto death. This semantic shift happened within the first centuries of Christianity, encoded permanently in the word itself.
The apostolic commission is fundamentally a witness mandate: 'You will be my witnesses in Jerusalem, and in all Judea and Samaria, and to the ends of the earth' (Acts 1:8). The apostles were specifically witnesses of the resurrection (Acts 1:22; 1 Corinthians 15:15). Revelation uses martus of Jesus himself: 'the faithful and true witness' (Revelation 3:14; 1:5) — Jesus whose very life, death, and resurrection was testimony to the Father. The first Christian martyr, Stephen, is explicitly called a martus (Acts 22:20). Revelation 6:9 honors 'those who had been slain for the word of God and for the witness they had borne' — the martyrs under the altar.
The 'great cloud of witnesses' in Hebrews 12:1 turns the OT Hall of Faith (Hebrews 11) into a heavenly stadium of martyres — those who testified to faith's reality and are now watching the race we run. Every Christian life is a testimony — a living marturia. The word 'witness' carries the weight of both court testimony (factual, specific, sworn) and lifelong embodiment (living in such a way that Christ is seen). The martyr tradition shows that the ultimate testimony is sealed with one's life.