A martyr is anyone who chooses to suffer, even to die, rather than renounce his or her faith or Christian principles. As every single one of us is called to this, we are all c…alled to be martyrs, as Our Blessed Lord calls each one of us to "take up your cross daily and follow me": (from St. Luke's Gospel) .
23 And he said to all, "If any man would come after me, let him deny himself and take up his cross daily and follow me. 24 For whoever would save his life will lose it; and whoever loses his life for my sake, he will save it. 25 For what does it profit a man if he gains the whole world and loses or forfeits himself? 26 For whoever is ashamed of me and of my words, of him will the Son of man be ashamed when he comes in his glory and the glory of the Father and of the holy angels. 27 But I tell you truly, there are some standing here who will not taste death before they see the kingdom of God.".
In the Church, however, martyr usually means a very specific thing: someone who actually dies for the faith, this is known as "red" martyrdom. Saint Therese of Lisieux was what is known as a "white" martyr, meaning that she gave up her life for the faith, but she was not killed for it (the meaning of red martyrdom). (MORE)