Thayer's Greek Lexicon

 1. a teacher
2. in the NT one who teaches concerning the things of God, and the
duties of man
  1) one who is fitted to teach, or thinks himself so
  2) the teachers of the Jewish religion
  3) of those who by their great power as teachers draw crowds
around them i.e. John the Baptist, Jesus
  4) by preeminence used of Jesus by himself, as one who showed
men the way of salvation
  5) of the apostles, and of Paul
  6) of those who in the religious assemblies of the Christians,
undertook the work of teaching, with the special assistance of
the Holy Spirit
  7) of false teachers among Christians