1) a sign, mark, token
1a) that by which a person or a thing is distinguished from others and is known
1b) a sign, prodigy, portent, i.e. an unusual occurrence, transcending the common course of nature
1b1) of signs portending remarkable events soon to happen
1b2) of miracles and wonders by which God authenticates the men sent by him, or by which men prove that the cause they are pleading is God's
Part of Speech: noun neuter
Citing in TDNT: 7:200, 1015
Neuter of a presumed derivative of the base of G4591; an indication, especially ceremonially or supernaturally: - miracle, sign, token, wonder.
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 σημεῖον; click on the second column to search for that grammatical form of the root σημεῖον; click on the third column to search for that word and grammatical form; click on the total to search for the root σημεῖον.