A sacrament occurs in a celebration when a deed done is so understood as to signify something that is received in a holy way. And so baptism and chrism, body and blood, are sacraments. These are called sacraments because, under the bodily skin of the thing, the divine power secretly works the salvation pertaining to those sacraments; thus on the basis of secret or holy power they are called 'sacraments'. These are fruitfully accomplished in the hands of the Church because the Holy Spirit dwelling in it works the effect in a a secret way....Thus in Greek it is called 'mystery', because it has a secret and concealed disposition.
[St. Isidore of Seville, Etymologies 6.19.39-42, my translation.]
I'd normally be hesitant to translate 'secretius' and similar words by 'secret', because (despite appearances) the word usually means 'private' or 'personal', not 'secret'; it's what gives us the word 'secretary'. Thus, for instance, the 'Vatican Secret Archives' sounds really confidential and mysterious in English, but the name literally just means that it is privately owned by the Pope -- it's basically the Pope's private filing cabinet (a library-sized filing cabinet!) for receipts, correspondence, and various miscellaneous documents pertaining to the Pope himself. It contrasts with the public archives of the curial offices. Here, however, while we could translate it as 'private' or 'personal', St. Isidore so emphasizes the fact of hidden power that this seems a rare case where the English word 'secret' is probably the best translation: a sacrament is a secret sacredness, a hidden hallowing.