Catechism of the Catholic Church §774 — excerpt
The Greek word mysterion was translated into Latin by two terms: mystenum and sacramentum. In later usage the term sacramentum emphasizes the visible sign of the hidden reality of salvation which was indicated by the term mystenum.
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. Augustine, Ep. 187,11,34: PL 33, 846