Objection 1. It seems that death is not essential to martyrdom. For Jerome says in a sermon on the Assumption (Epist. ad Paul. et Eustoch.): "I should say rightly that the Mother of God was both virgin and martyr, although she ended her days in peace": and Gregory says (Hom. iii in Evang.): "Although persecution has ceased to offer the opportunity, yet the peace we enjoy is not without its martyrdom, since even if we no longer yield the life of the body to the sword, yet do we slay fleshly desires in the soul with the sword of the spirit." Therefore there can be martyrdom without suffering death.
Objection 2. Further, we read of certain women as commended for despising life for the sake of safeguarding the integrity of the flesh: wherefore seemingly the integrity of chastity is preferable to the life of the body. Now sometimes the integrity of the flesh has been forfeited or has been threatened in confession of the Christian faith, as in the case of Agnes and Lucy. Therefore it seems that the name of martyr should be accorded to a woman who forfeits the integrity of the flesh for the sake of Christ's faith, rather than if she were to forfeit even the life of the body: wherefore also Lucy said: "If thou causest me to be violated against my will, my chastity will gain me a twofold crown."
Objection 3. Further, martyrdom is an act of fortitude. But it belongs to fortitude to brave not only death but also other hardships, as Augustine declares (Music. vi). Now there are many other hardships besides death, which one may suffer for Christ's faith, namely imprisonment, exile, being stripped of one's goods, as mentioned in Hebrews 10:34, for which reason we celebrate the martyrdom of Pope Saint Marcellus, notwithstanding that he died in prison. Therefore it is not essential to martyrdom that one suffer the pain of death.
Objection 4. Further, martyrdom is a meritorious act, as stated above (2, ad 1; 3). Now it cannot be a meritorious act after death. Therefore it is before death; and consequently death is not essential to martyrdom.
On the contrary, Maximus says in a sermon on the martyrs that "in dying for the faith he conquers who would have been vanquished in living without faith."
I answer that As stated above (Article 2), a martyr is so called as being a witness to the Christian faith, which teaches us to despise things visible for the sake of things invisible, as stated in Hebrews 11. Accordingly it belongs to martyrdom that a man bear witness to the faith in showing by deed that he despises all things present, in order to obtain invisible goods to come. Now so long as a man retains the life of the body he does not show by deed that he despises all things relating to the body. For men are wont to despise both their kindred and all they possess, and even to suffer bodily pain, rather than lose life. Hence Satan testified against Job (Job 2:4): "Skin for skin, and all that a man hath he will give for his soul" [Douay: 'life'] i.e. for the life of his body. Therefore the perfect notion of martyrdom requires that a man suffer death for Christ's sake.
Reply to Objection 1. The authorities quoted, and the like that one may meet with, speak of martyrdom by way of similitude.
Reply to Objection 2. When a woman forfeits the integrity of the flesh, or is condemned to forfeit it under pretext of the Christian faith, it is not evident to men whether she suffers this for love of the Christian faith, or rather through contempt of chastity. Wherefore in the sight of men her testimony is not held to be sufficient, and consequently this is not martyrdom properly speaking. On the sight of God, however, Who searcheth the heart, this may be deemed worthy of a reward, as Lucy said.
Reply to Objection 3. As stated above (123, 4,5), fortitude regards danger of death chiefly, and other dangers consequently; wherefore a person is not called a martyr merely for suffering imprisonment, or exile, or forfeiture of his wealth, except in so far as these result in death.
Reply to Objection 4. The merit of martyrdom is not after death, but in the voluntary endurance of death, namely in the fact that a person willingly suffers being put to death. It happens sometimes, however, that a man lives for some time after being mortally wounded for Christ's sake, or after suffering for the faith of Christ any other kind of hardship inflicted by persecution and continued until death ensues. The act of martyrdom is meritorious while a man is in this state, and at the very time that he is suffering these hardships.
Source: Summa Theologica (New Advent)