Thayer's Greek Lexicon

 1. a sign, mark, token
  1) that by which a person or a thing is distinguished from
others and is known
  2) 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