Thayer's Greek Lexicon

 1. summoned, called to one's side, esp. called to one's aid
  1) one who pleads another's cause before a judge, a pleader,
counsel for defense, legal assistant, an advocate
  2) one who pleads another's cause with one, an intercessor
1b1. of Christ in his exaltation at God's right hand, pleading
with God the Father for the pardon of our sins
  3) in the widest sense, a helper, succourer, aider, assistant
1c1. of the Holy Spirit destined to take the place of Christ
with the apostles (after his ascension to the Father), to
lead them to a deeper knowledge of the gospel truth, and
give them divine strength needed to enable them to undergo
trials and persecutions on behalf of the divine kingdom