From the Merriam-Webster on-line dictionary:
Main Entry: ¹mar·tyr
Pronunciation: \ˈmär-tər\
Function: noun
Etymology: Middle English, from Old English, from Late Latin, from Greek martyr-, martys witness
Date: before 12th century

1 : a person who voluntarily suffers death as the penalty of witnessing to and refusing to renounce a religion
2 : a person who sacrifices something of great value and especially life itself for the sake of principle

On this day, August 9, 1942,
Sr. Teresa Benedicta of the Cross (Edith Stein) became a martyr for the faith in the Auschwitz death camp.  She continues to witness today.