I didn't choose that definition. It is out of the Strong's concordance.
Again here is what it says.
3466. musterion moos-tay'-ree-on from a derivative of muo (to shut the mouth); a secret or "mystery" (through the idea of silence imposed by initiation into religious rites):--mystery.
Again here is what it says.
3466. musterion moos-tay'-ree-on from a derivative of muo (to shut the mouth); a secret or "mystery" (through the idea of silence imposed by initiation into religious rites):--mystery.
28. 77 μυστηριον ου n:the content of that which has not been known before but which has been revealed to an in- group or restricted constituency- secret, mystery. υμιν δεδοται γνωναι τα μυστηρια τησ βασιλειασ των ουρανων the knowledge of the secrets of the kingdom of heaven has been given to you MAT. 13:11. There is a serious problem involved in translating μυστηριον by a word which is equivalent to the English expression mystery, for this term in English refers to a secret which people have tried to uncover but which they have failed to understand. In many instances μυστηριον is translated by a phrase meaning that which was not known before, with the implication of its being revealed at least to some persons. (LouwNida)
[3696] μυστηριον mysterion 28x a matter to the knowledge of which initiation is necessary; a secret which would remain such but for revelation, Mt. 3:11; Rom. 11:25; Col. 1:26; a concealed power or principle, 2 Thess. 2:7; a hidden meaning of a symbol, Rev. 1:20:17:7 (MED)
g3466. μυστηριον mysterion; from a derivative of μυω muo (to shut the mouth); a secret or “mystery” (through the idea of silence imposed by initiation into religious rites):— mystery.
AV (27)- mystery 27;
hidden thing, secret, mystery generally mysteries, religious secrets, confided only to the initiated and not to ordinary mortals a hidden or secret thing, not obvious to the understanding a hidden purpose or counsel secret will of men of God: the secret counsels which govern God in dealing with the righteous, which are hidden from ungodly and wicked men but plain to the godly .
in rabbinic writings, it denotes the mystic or hidden sense of an OT saying of an image or form seen in a vision of a dream (EXPANDED STRONGS)
But I'm sure the above will be discarded in favor of one's agenda.