1) hidden thing, secret, mystery
1a) generally mysteries, religious secrets, confided only to the initiated and not to ordinary mortals
1b) a hidden or secret thing, not obvious to the understanding
1c) a hidden purpose or counsel
1c1) secret will
1c1a) of men
1c1b) 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
2) in rabbinic writings, it denotes the mystic or hidden sense
2a) of an OT saying
2b) of an image or form seen in a vision
2c) of a dream
Part of Speech: noun neuter
Citing in TDNT: 4:802, 615
From a derivative of μύω muô (to shut the mouth); a secret or "mystery" (through the idea of silence imposed by initiation into religious rites): - mystery.
These two sites give similar information, with the definition from several dictionaries and statistics on the use of the word.
Perseus Digital Library
University of Chicago's Logion lexicon
Thayer's dictionary plus other information.
From this site's dictionary (in Italian)
Click on the first column to search for that word as a form of the root μυστήριον; click on the second column to search for that grammatical form of the root μυστήριον; click on the third column to search for that word and grammatical form; click on the total to search for the root μυστήριον.