Catechism of the Catholic Church §2473 — excerpt
Martyrdom is the supreme witness given to the truth of the faith: it means bearing witness even unto death. the martyr bears witness to Christ who died and rose, to whom he is united by charity.
This translation is under copyright, so only the cited words are quoted here. Follow the link for the full text. Translation: English translation of the Catechism of the Catholic Church, 2nd ed. (Libreria Editrice Vaticana), as posted at vatican.va.
Also cites: St. Ignatius of Antioch, Ad Rom. 4, 1 SCh 10, 110