Thayer's Greek Lexicon

 1. it is necessary, there is need of, it behooves, is right and proper
  1) necessity lying in the nature of the case
  2) necessity brought on by circumstances or by the conduct of
others toward us.
  3) necessity in reference to what is required to attain some end
  4) a necessity of law and command, of duty, equity
  5) necessity established by the counsel and decree of God,
especially by that purpose of his which relates to the
salvation of men by the intervention of Christ and which is
disclosed in the Old Testament prophecies
1e1. concerning what Christ was destined finally to undergo, his
sufferings, death, resurrection, ascension

For Synonyms see entry G5829